I've never seen a pet despawn in arena, I run 2v2 with a druid as well, and it just seems we can't coordinate cc like we use to be able to, to get the lock down. Are you sure going on the paladin is the best choice in that scenario?
We've had the most success with killing Paladins over Warlocks. Going for an SL/SL lock with a VW out is a nightmare, so imo your best bet is to try and take out the Paladin. He'll take about as much damage as the lock but doesn't get +20% healing taken (or have siphon/drain life) and your Warrior can interrupt the heals.