How many beats will result?

Zero, or I suppose you could say one when both of the forks are struck. The frequency of a tuning fork correlates to the note that is heard after it is struck and not to any sort of rythm. Also of note is that these two frequencies are not of harmonic resonance So it would just sound like two seperate notes being held. Finally, and more importantly of note, I tried to find it on wikipedia but couldn't and I don't have an EQ in front of me but I don't think that 20 hz is even in the range of human hearing and if it is it is a very low note. If you had a good sub woofer you might feel a vibration from it.