For most things, adding more won't make a difference. I've done 2 or 4 whole chickens and it doesn't matter.
As for 2 turkeys, which smoker? In a Smokette or 50 it will be a little longer, but not much, I don't think. But the turkeys will have the final say
But think about the butts, they're loaded with moisture and putting two of them in there usually does add some time (depending on the smoker, like a Smokette). When I've filled the smoker with butts and briskets, it does take a little longer, but that's in the smaller CS. Not a problem in the bigger units.
Smokin' is an art not a science, so everything we say here is a generalization. MOST things it won't add time, some things it does.