if it does as an infrequent happening anyway, you'd use it as your white swing landed, your main hand white swing or just after they both land if they are in quick succession both weapons with close to the same speed would be good likely.
Slam is very high dmg to rage ratio so it's pretty sweet even if it clips the start of your swing slightly.
If it does, it's probably not intended to, i suspect; much like how a full maelstrom weapon stack (=instant cast) LB/CL/whatever currently resets the swing timer.