I think the simplistic theory is that you wont be able to mark both of them so if you mark messi that frees up space for neymar and vice versa the problem is that in general they both like to occupy the same spaces of the pitch so it will be interesting to see if they can adapt their games around each other as they may just get in each others way.
What i find weird about this transfer is that it doesnt address their main problem of needing a central defender and another top striker which will probs be another 50m at least to that end you can see villa going and maybe pedro, the problem with alot of barcelona players i.e thiago and pedro is that they would walk in to any other team on the planet and the thrill of sitting on the bench for barcelona can only last for so long. If you assume they have a front four and 2 of those spaces are occupied by messi and neymar then that leaves iniesta, xavi pedro, tello thiago and youngsters fightin for very few spots and someone may have to go.