Tags won't have any affect on this, tags are just applied to existing entities that have already been mapped/organized. For there to be any possibility of merging or combining them they must first be in the same process group. Which likely means using process group detection rules. Process groups are a hard boundary for services.
Once in the same process group you could use merging (being deprecated) or preferably service detection rules via the API:
Depending on what the goal is you might find though that after combining them into the same process group that the underlying services are combined at that point too after they are restarted.
I was hoping that this would work, as it is part of the attributeType. But reading your reply I see this is more a placeholder.
This will create multiple services (from the different PG's with that same label) with the same name,