I must confess that this question of mine is quite old, and I don’t remember how I resolved my problem (maybe with a recursive loop).
But, for sure, the loop nodes have changed since 2012 and now you have options like “Allow variable column types” or “Allow changing table specifications” that should allow you to concat or merge 2 virtual output in one before the loop end node. And then, based on a tag that you should have added before the concat, you can split again the 2 flows with a Rule based splitter. Even, you can validate that both final table are correct with a Table validator. Not very convenient but it will work.
In a general way, my advise is to find a way to avoid loops. Maybe one or more actions in you loop can be done through a java snippet or Python Script node, or simplified with a goupBy node?
Do not hesitate to post your wf if you want more advise.