Deadlock when trying to follow Combination Lookup/Update by an Insert/Update?

I need to use one step to find/create an item's unique identifier, then use the Insert/Update to set some other values that are not part of the unique key.

This results in a deadlock, probably because all steps run in parallel.

What's the correct way to do this? The main problem is that the Combination Lookup/Update requires to specify only "key fields" but the values I want to update are not all keys.

Thanks