I cannot think of any way how this would be possible.

A reorg as Xynerise already explained can only invalidade (orphan) the shorter chain, but this would not be a merge.
A reorg in this case would be a disaster. The losing chain would throw back all transactions to the mempool.
This would first of all spam the mempool (depending of how many transactions happened parallel in the forked chain) and I guess it would invalidate most transactions which references to any double spend coins.

The main problem here are the double spends.

When you spend some coins on fork 1 and on fork 2, which transaction after the merge would be valid?
It cannot be both, it would brake the consensus rule of double spending. So it only can be one of them.

And if it can only be one of them someone need to chose which one.
And if the decision was made which one is valid, that mean that ALL transactions from this side of the fork need to be valid and ALL transactions from the other side of the fork need to be invalid. And now we are back at the reorg.

You cannot chose some transactions from fork 1 and some from fork 2.