When your speakers are 8Ohm you still get 10W.
The signal is coded as voltage. Higher voltage
swing requires more current (use Ohm's law).
If your amp can deliver the current it is fine,
if not the signal get distorted.
If you use 4Ohm speakers you should get more power
with 2 zens in parallel (4Ohm speakers require twice
as much current for the same voltage). One zen
could get problems with it but with two you should be
ok.
If you use 2 zens with 8ohm speakers you
still get something in return, namely,
lower distortion.
Greg