Not enough fingerpints on the Galaxy S6
If you haven't already figured it out, here is how.

Tap Add Fingerprint.
Do your thumb until it reads about 50%, then just switch to your forefinger for the next 50. Heck, you can even alternate, it doesn't know the difference (but of course, it does). That's all. Maybe do your less used finger after the 80% point.

You can do multiple fingers under one name, but what I don't know for sure is one thing... if the finger scanner is dynamic. Meaning, the more you use it every day the better it gets (Fails less often). If not then I would limit it to 3 fingers per "print". If it is then you can do as many as you need.