yeah I believe they will still strike last, as the Strongman, skill specifically mentions the Two handed weapons strike last rule not the Obisidian weapons one, whilst something like the "Great Axe" mentions it works with Strongman.
but for the strike last part, i dont believe it would stack, so some with a Double handed weapon and someone with a double handed obsidian weapon would go off initiative order.
this is the same as Strike first, multiple instances of it do not give you an advantage, say a charger with a spear vs a defender with a spear Or lightning reflexes or something like that, the combat goes off initiative.
So any amount of Strike first vs Strike first = Initiative
strike first vs regular = Strike first goes first
Strike first vs strike last = strike first goes first
"Regular" vs "Regular" = Initiative
Regular vs strike last = regular goes first
Strike last vs Strike last (no matter how many instances of it they each have)= Inititave.