Two buttons seem to be higher DPS.
But one button is simpler to implement.
Your choice really.
Or if you use something like IS Boxer, which can alternate output keys from your one button spam, then the two button is superior.

I'd really consider a Shockadin like build for the Pally.
That gets you support DPS for the DK.
And heal ability for both toons, which should reduce downtime a lot.
And with a dual-spec, including Blood, get you Tank + Healer options.