The jersey sales are one option and perhaps Barcelona sponsors would also help finance such a deal, but there will be financial limits even with sponsors involved. But maybe Messi already thinks about the time after his career and wants to make a step towards Barcelona now. But will he really forego the opportunity to make a couple of hundred of millions going abroad?
But, if somehow Barcelona able to have them in their team at same time then we can expect amazing work and games against top team in La Liga and Champions League as well as hype is going nothing is clear because no one can predict where Messi is going to land for the next season and FFP problem allow Barcelona to have this player back into team.
If everyone were to play to the best of their abilities, Messi, Mbappe and Neymar should be more efficient than Lewandowski and Messi. Whatever the outcome may be of the next transfer window, Messi and Lewandowski could work out, but only for a year or two. You usually don't want two people approaching 40 years of age to be the pivotal point of your game.
Barcelona has to be planning for the future as well. Messi can assist and he can still decide games, but as much as I like him and I also think that Lewandowski is great, the two playing together would still be a transition period into something new. Messi could hep with building something new though.