I probably shouldn't be giving you tips, because if I don't you might do something as pointless as a transfer like this and give me a chance to catch you in the league. That said, you're doing so well in the early season for a second year in a row, I really don't want you to screw it all up again either!
I'm assuming here you have 2 free transfers. Transfers are precious. You get 38 of them. The should be used exclusively to improve your team. Whether that be through removing injured players, underperforming players, suspended players, every time you do one it should be done to maximise your points potential for the next few gameweeks. I personally don't like making transfers unless I want those players for the next 5 games minimum.
Now the question here is, does swapping a Manchester City and a Liverpool player for a Manchester City and a Liverpool player maximise your points potential. Without even naming specific players, the answer to this is a resounding no. When you look at the players you're wanting to change and ask 'Does this improve my team' the answer is also no.
If you're concerned about Aguero being subbed off after 60 minutes, that's fair enough. Stirling's playtime is equally as unassured as Aguero's though. They will both start most games, but are highly likely to be subbed early and rested on the odd occasion. Those are the inherent risks of having a premium City player. They are also incredibly likely to score very similar amounts of points over a season. The same can be said of Mane and Firminho, who are both players who go through patches where they either do nothing, or get explosive scores. All 4 of these players will probably score close to or more than 200 points this season. I doubt there will be more than 40 points between the highest of them and the lowest. You don't want to waste transfers switching these kinds of players about. That's not really what transfers are for.
Over a subset of 5 or 6 weeks, there may be a huge swing in favour of one over the other. This is true. Just look at the start of the season. Mane scores 4 goals in 4 games. Then 0 goals in 4 games. Now you want him out. Firminho is still 14 points behind him, even with Manes blank patch. Neither of them are exactly in form, but they are playing for a goalscoring team. Could Firminho even that up? Possibly. Could Mane pull further ahead? Also possible. You just can't call which of the 3 attackers in a freescoring side like Liverpool are going to perform. It's all just variance. Luck. There is nothing you can do about that. What you do know is that eventually they all will perform. Now, you could still go ahead and make this transfer and do amazingly out of it. I'm not saying you couldn't. But, if you did, that would make it a lucky decision, rather than a good decision. And whilst luck is important in FPL, luck is not how you do well in FPL.
A good decision, if you are worried about something like, say, Aguero's gametime, would be to see how downgrading him to somebody who might match his points output over the next 5 games, like Arnautovic or Mitrovic, and upgrading somebody in your team who just isn't performing and can't be relied upon for points, like Neves or Moura. You say that Mane is not performing, but what about Moura? He has blanked in the last 4 games as well, and he is playing for a team that is currently struggling to score and he is far from guaranteed to start in. You have Neves in your team. He has blanked for 7 games in a row. He's an absolutely incredible footballer, but he will be lucky to get 100 points this season. He is not a fantasy football asset. Mane is. He will score points. If Aguero is troubling you that much, and he needs to go, you could do Aguero to Arnautovic and Moura to Stirling. One of the best ways of making transfers is just putting the two players next to eachother and asking yourself 'Who scores more points?' Aguero/Neves or Stirling/Arnautovic. Over the next 5 games, who scores more? I know how I would answer that one.
That's a transfer that improves your team and could get significantly more points over the next few weeks than playing the City/Liverpool 'who will score this time?' lottery. You have a pretty healthy squad, but this is just one of many things you could do.You could look at shifting some money out of your defence to upgrade your midfield deadweight. Like last season, I think the high priced defenders getting all the points at the start of the season phenomenon will die out soon, as midfield/forward partnerships start clicking. Keeping 4 high price defenders may prove detrimental with the Wolves defence looking incredibly stubborn and mid priced midfielders like Richarlison and Maddison doing the business. And you don't even have a City defender in your 4 premium defensive line up! As an Arsenal fan I would not recommend keeping Monreal, our defence is going to leak goals for the foreseeable future. Tottenham have hard fixtures coming up and rotate their fullbacks so Trippier is always someone it's risky to have. Alonso is a solid pick but arguably not worth the money. You could get rid of one of them for a Wolves defender, or someone like Holebas, upgrade Neves to somebody who will actually get points and still have a really strong team and bench. Again look at the options you have. For example Trippier/Neves or Richarlison/Doherty, who scores more? That one is closer to call, but I still think it's the latter. That's just one of many pairings you could look at.
You could do any of these things and they would be better decisions than just switching players from the same teams around. Second guessing yourself is the best way to do badly in this game. If I was in your position, as you can probably guess, Neves and Moura would be my priority transfers out. Who you get in for them, how long you take to do so and whether or not you potentially downgrade an expensive player to upgrade them is up to you, but getting rid of those two is your best bet for keeping or improving your rank. Imagine if you did this transfer you're asking about and it fails miserably, and then Moura and Neves continue to blank and your rank plummets. That's just as likely as it going well. Look at your decision to get Lacazette in. That was an absolutely fantastic transfer. You got him in at exactly the right time, he looks like he'll keep scoring goals and now everyone wants him. If you want to keep doing well, you need to make decisions like that one. There is definitely a defender/midfielder combo that will get more points than Neves + one of your premiums. There is definitely a midfield/forward combo that will get more points than Neves/Moura and Aguero. There are probably even single transfers from either of those players that will score your more points in the long run. You want to be looking for those, instead of doing the Liverpool/City hokey-cokey every couple of weeks.