I don't wear bifocals but normal glasses are a bit of a pain when rangefinding as you're generally looking through the glass at the edge which may not be as close to your proper prescription as it is in the middle. Also not good with a scope enhancer either so I use disposable contact lenses for shooting.
Have you considered a magnifying lens mounted on the scope? You can rig up something yourself attached to a scope mount from a butchered pair of cheap reading glasses, or buy something from Maestro design, or there may be others.
using bifocals you'd have to be careful not to rangefind with the wrong part of the lens, and I imagine varifocals would play havoc with rangefinding!