Zilean, Time Mage
Card Text & Abilities
Once each turn, if you would play a token unit while I'm at a battlefield, you may play that token and an additional copy of it instead.
Official Rules FAQ & Clarifications
1 Official FAQLet’s walk through this step by step. Remember, we are first going to add to the chain and then we will work in reverse order to resolve these things.
Remember, you finalize all of these items in the order that they were added to the chain, starting with the two tokens that are placed on the chain in the order of their controller’s choosing.
So now we work through the above ordering in reverse:
By this point, even if the two tokens become copies of Zilean, there is no subsequent play game action involving tokens to apply their replacement effects to, so they will not be able to cause any further tokens to be produced.