Possibly giving only ambush and backstab an extra Mulstrike chance (just like PvE).
Only concern I have over it, is that if it ends up being too much extra damage.
Even if you buff Multistrike for Backstab and Ambush it wouldn't affect that much.
You need even more Ruptures for that and you dont have Combopionts left for other stuff.
In PvE Rupture has a ~10sec duration sometimes