I wanted to ask if a slow offhand is also a viable option for Sub because i see many Sub Rogues using it.
Also Ambush Tooltip tells me (527% + xxx if a Dagger is equipped). Do i need to have a Dagger in both slots or only in MH to get the bonus there?
The reason some PvP rogues do that is so they can play combat (2.6/dagger) and subt (dagger/2.6) with the same set of weapons. This lets them put all their points into gear while still having spec flexibility. It's not optimal for either spec, but the loss is minimal. Once they're fully geared, they usually pick up another OH dagger for subt and another OH 2.6 for combat.
The reason PvE rogues do that is they have a 2.6 that's way better than their dagger OH. The difference in weapon damage has to overcome the difference in poison procs for it to be an upgrade.