You can't make a union of both lists, because the lists are mutually exclusive. Each of them only exists in one case depending on which side of the split is taken.
It's unclear to me what you're trying to do exactly with this flow, but I would consider the making the part where the list is created a submicroflowflow that returns the list that you need. Then you likely won't need your union at all.
Depending on the use case, it's probably also better to create the lists outside of the flow and instead change the lists inside the flow, but you’ll have to judge that based on your case.