You can do it even now on pvp servers. All you have to do is find one that’s interested about this fight, both of you order your thralls attack nothing and both of you order to attack the opponents thrall. The fight won’t stop as long as both of you are in the render of fight and nobody tries to help the defeated thrall. But no matter what you’ll do this fight will never be fair. The reason is that the thralls attack with rng orders, their AI to combat is like the npcs, stupid. So the winner will be the most fortunate not the most capable.
Knowing this, will you drop your thrall in a fight?
But the greatest reason i don’t agree for this to happen is because pve should and must stay pve, any conflict between players exists on pve-c and pvp servers, leave pve alone please.