It's fine to run more than one dedicated circuit. Run them on the same rail at the main. If you run a pair of 20s run BOTH on L1 or L2 don't distribute the load between the two. That is a good way to pick up noise.
To tell the truth these days with class D amps and high effeciency speakers a single 15 or a pair of 15s will work without a hitch.. Same rail, separate breakers.
Regards