2 ohm stable means what it says in the owner's manual. If it's 2 ohms stable in stereo that means you have to keep it at 2 ohms or greater per channel.
As to which would sound better, 8 speakers on one amp or 8 speakers on 2 amps, I think it would just be loud either way. You technically should get a bit better sound quality out of the 2 amp setup due to less THD, but it's still a lot of speakers. More speakers is not always better for sound quality. It seems you're more concerned about getting loud.
Is there a certain combination of speakers and amplifiers you're looking to use?