Signing them and then trading them is an idea, doesn't sound like a great one though. If their cap hits are enormous that will limit any return and that's not even getting into the NMC's they will undoubtedly be demanding. It will also be another 2 years before Marner's next contract kicks in, it feels we're just screwed no matter how we we play this. Unless of course M&M are willing to sign reasonable deals, doesn't seem likely though.
Anyone can be in a cup final, it's a matter of being on the right team. I don't see an overpaid M&M duo leading this team to a cup final, that's what I would say.