Add Multiple Child Records at one time to a join table and have the records linked to both parent tables

It seems to me that this would be pretty straightforward with a Pipeline. He would figure out what your trigger is and that would be a on new event type trigger.

Then you would search the tags. Where is the tag number is between the starting tag in the ending tag

then you would have a For Each loop to create the join table records. 

In theory, it is more efficient to create a bulk upsert right after the trigger step and then add rows  to the bulk upsert and then at the end of the process commit the upsert, but if you're adding say 10 or 20 records, it will be fine be to do the for each loop to create the join table records.