Well I have to disagree here!
Doesn´t need to be on same engine, what you would need as a user is both engines installed at your client.
What the game designers would need is a compatible database, so the skills and stats can be merged, this not only for avatars also for items. This is the main problem!
How do you want to merge a laser sniper rifle into a fantasy role play game.
Well still possible! Its a ranged weapon that does mainly burn DMG, so if you telport from EU into WoW your laser rifle becomes a fire bow. Teleporting back, the fire bow becomes a laser rifle.
With a lot of work, databases could be adjusted and the merge routine could do it all. The loading process for such cross game teleport could take a while, the more items you own the longer it would take, but not impossible to do.
I wouldn´t say its not doable, just a lot of work and game designers would need to work together to generate compatible data base for items and avatars.
The engine used doesn´t matter at all, as it can just be loaded clientside when you use cross game teleport.
The idea iteself is awesome, but unlikely to happen for some reasons, and the main reason is money!
Each game developer want to make money, weather by monthly subscriptions or by premium things sold.
What would it mean for game developer when high end player comes to his game and has everything without paying a dime? NO income, just because other game developer already earned the money this player invested to develop his avatar.
So the real problem isn´t the technical issus, the real problem is the money the game developers want to earn!
Even worse considering a cross game teleport from a subscription based game into a RCE game.
Consider some WoW high end player teleports into EU, sells out and withdraw all the PEDs made, MA would go bankrupt in no time
Thats why it won´t work!