The OP is if you can only keep one, so to say both is kind of missing the point. Assume a team offered a good young player a decent role player with some salary, plus a good draft pick and then said we will make the deal for either Pierce or KG, you pick, and also assume that the other retires at the end of the season.
I would send out KG, not for basketball reasons but PP played his whole career here so for him to retire here would mean more.
I think it could only go one of two ways:
For KG only, he doesn't waive to leave PP and the C's. I think he chooses to go down fighting with Paul.
If the offer is for Pierce straight up, it has to be worth losing both Pierce and KG, since I think KG walks w/o Paul. That kind of deal ain't happening.
If its an offer for BOTH Pierce and KG to go to the same team, then who knows...
But the question is a non-starter, since I don't think it's even a remotely likely scenario.