There are times when two players (or even other kinds of Entities) need to trade an object or perform some other action that is critical to occur transactionally. Generally, this is meant to mean that no matter what kind of error occurs, the transaction occurred or did not occur. E.g. I paid you 10K, and got a gold bar.
Support for transactions needs to work across server hosts. It is a very disruptive experience to be able to interact in one spot, but not 2 feet to the left. You have to assume that the players can purposely crash one or the other host at the worst possible point in the interaction (like Byzantine Generals). Assuming you can trust you database, the idea is simply to get both sides of the interaction saved to the DB simultaneously (i.e. in one DB transaction).
What if the two Entities are on different hosts? You have to simultaneously change remote variables and get the persistence request from two places to the DB, blah, blah, distributed handshake, Byzantine...brain fry. They do this in Cobal for banking systems. How hard can it be? Well it is too hard to be worth it (if you include failure/backout/retry, etc), even if you think it would be a fun challenge.
So here is a fantastic consequence of the non-geometric load balancing mechanism we've been talking about. Each simulator is single threaded (if you use multiple threads, you have multiple simulators). An Entity Behavior will execute without preemption. So straight-line code will run to completion, a DB save request can occur, and all is good. All you need to do is get both Entities onto the same simulator, and run that straightline code and co-persist the two local Entities (i.e. send the db save request with both entity's states). Boom. Done. Either the transaction makes it to disk or it doesn't. Not just half. This scenario is the origin of a lot of dupe bugs you've heard about.
The fact that migration policy and mechanism are separate means that adding a new policy is easy. E.g. migrate the guy I'm about to interact with over here (or me over there). Once it finishes, the transactional behavior can be scheduled. If things are a little crazy, it may take a while, but it won't fail and start giving away money. And it won't tell the user the transaction succeeded when it didn't.
Clearly, this is not something you want to do all the time. It would be too slow. Just for the stuff that *really* matters to the users.
If you don't like the thought of your Entities migrating around, build an Escrow Entity. Place one side of the transaction into it and copersist. Migrate to the other side, place the other half in and copersist. Move out the half, persist, migrate, move out the other half, persist, done. If there is a failure at any point, the Escrow object is known about by the DB and it can continue, or return the goods. Just like a Lawyer, but not as expensive.
Either of these approaches can support multi-Entity Transactions.