My group actually does this too. When the 2nd dreadnought pops I do a quick threat-grab and pop Hallowed Ground. As soon as mine drops, OT pops his HG. As that is ending, I'll activate Tank LB to help keep group alive through the last bit of phase 6. Usually our biggest issue has been keeping everyone alive through the non-dreadnoughts, whether it be hate/dmg issues. Its become a guaranteed win once we are down to just the 2 dreadnoughts left.