Just checking my own research, but the RD2 can count at a rate up to 10Meg, on a 20Meg clocked part ? I was looking in the timing diagrams, and I can't see it explicitly stated.
Well, it can if you are using the internal oscillator. Possibly it will need either polarity at 50ns since the oscillator is symmetrical but otherwise I do not see a problem. For the "regular" '51 there is no "timer external clock" specification either and there 1MHz for a 12MHz part hold true.

Erik