The ADC has 10 bits of resolution, so the result doesn't fit into a single 8 bit register. The most significant bits of the result are in ADRESH. I think you'll find that you get 255 at 25% and 0 at 26% as well.
If you clear the ADFM bit, 8 bits of the result go into ADRESH, with only the two least significant bits in ADRESL. You can then just use ADRESH.
To use two pots, connect to RA0 and RA1. You can only read one at a time, each reading takes much less than 1ms.