With a Summoner you want the tanks to drag the mobs slightly near each other so Bane hits everything. SMN should nail everything with Bane and focus one of the soldiers at a time. The physical DPS should switch to the soldier that gets stunned and burst it down. The SMN should switch to the other soldier when the first is stunned (do not spend time bursting it down) and repeat. Done.
Edit: Bards should preload/maintain DoTs on both knights so they have more Bloodletter spam for bursting.
Also, the DRG should immediately ID-DE the soldier when the switch occurs.