If the offtank kites properly, he should be able to take no damage from the soldier and knight except when they pull him back in.

When the soldier and knight pop on wave 6, have the OT stands at one of the drop spot, use Fight or Flight as soon as he sees both targets show up on the enemy list (they show up on the enemy list before actually dropping down to the arena). As soon as they drop, flash the one that he is standing on, target the further away one and Shield Lob right away.

After initial aggro, the OT should run in an oval shape at the opposite end of the arena from the two DNs and just use Shield Lob/Tomahawk on the soldier and knight, this should be more than enough to keep hate from both targets. He can flash whenever he is pulled in by their draw-in attack and continue running in the oval loop to kite them.

This will alleviate a lot of healing stress from the healers allow them to focus on the MT who is tanking two DNs. When you are ready to DPS them down, the OT stops running and should have more than enough CDs to tank through without much additional stress on the healers.