Slightly depends on the spec. Instant on mh and wound on oh would probably be the best if you haven't specced to deadly brew. In case you did spec to it, Double instant.
On instances / soloing elites n stuff. instant in mh and deadly on oh.
(this is presuming you did spec full poison talents.)
like above poster it really depends on your spec. Imp poisons and deadly brew double instant. Just imp poisons MH instant oh wound so you have poison debuff for mutilate. No imp poisons double wounds.
As for what I used around lvl 65-67 I switched to instant main hand and deadly offhand so I could use envenom. Even though I wasnt stacking up to 5 stacks right away envenom buff was helping kill mobs faster then eviscerates were.
Its been a while since I have leveled but I went with what ever the hardest hitting poison granted to me on which ever level I was and that poison goes on the SLOW weapon with the FAST weapon holding Deadly poison.
DO NOT get into the MH OH mentality its the SLOW HAND/FAST HAND mentality that you need, as of rite now I have a FAST MH with DEADLY on it and a SLOW OH with INSTANT on it.