Our DPS don't swap at all. We just leave melee on Toth the whole time. Even with double damage, it's easy to heal through on Hard Mode. The only damage the melee are taking is from Smash in that phase, and that's not difficult to heal through. Your tank should have a CD going through the Berserk, and the Zorn tank doesn't take much damage at all during that phase, so healers can pretty easily focus on the Toth side.
This. The guys on Zorn aren't going to take much damage during that phase. Just as well, you said you were getting hit by rocks. This SHOULD NOT be happening. Watch the Methodical video; the bosses should be far enough apart so that the rocks won't hit anyone on Toth (remember, it's only a 30 meter range).
We had two sage healers and would have the healer on Zorn drop an aoe on the two melee on the Toth side during berserk (Bear shirt). Also, being a sentinel with your aoe damage reduction, your healer shouldn't have a problem keeping you up. He should be dropping aoe heals on Toth. We run with a Vanguard DPS and a Sentinel as well. The vanguard will sometimes have to pop his bubble and adrenaline rush, and if the healer really needs help we have him run out and dps Zorn, but this is highly unwanted. The dps need to be putting their balls to the wall throughout the whole fight.