Hmm... interesting idea. If we're talking sine waves with complete cancellation when they're 180 degrees out, maybe an envelope follower can be used? If the beating is slow enough obviously.
EDIT: It should work somewhat with square waves and triangle waves too because symmetrical waves only contain odd harmonics so they all cancel each other out at 180 degrees. Saws might work too if you invert one of them (as jimmy_p suggests), it'll produce a pulse wave with a width that cycles at a rate determined by the frequency difference between the two oscillators. If I understand correctly how envelope followers work then that should get a changing level too (high as the PW is 50% and low as it crosses over from 100% to 0%).