Some pretty good comments above. A couple other things come to my mind.
Aspherical lenses halp to as they remove the refraction SwampDog was talking about. The presription carries all the way to the edges. Similar to good binocular being clear right to the edges.
Your lenses are going to have to have antireflective coating on them. Otherwise you are likely to see two sets of pins (if you shoot pins). I know this because I have to get my new glasses coated.
You shouldn't have much of a problem with the bifocals because you don't look through them anyway when sighting. Bifocals are for close work like reading and have nothing to do with aiming at target some yards away.
As Lady Forge said you may find you have to turn your head toward the target more directly. Depends on how much you turn your head now. Frankly this shouldn't be an issue as this is how you should be shooting in the first place. This also may make it necessary to slightly shorten the bow's drawlength a little and you might have to move the sight in toward the bow a touch. It all depends on what your anchor is like now.