This is not a very straight forward implementation of either the multiple insert or MRT. If you would like to get all of this info into a single new table it will require a bit of custom development along with the multiple insert server behavior. This is because the insert is not designed to work with existing records and selecting the record to insert is not a part of the server behavior.
Since you have the information already recorded in two unique tables you should just be recording the id's from these two tables into your relational table. This way whenever you want to get at this info you can just use the relational table in a join query to create the recordsets or views of the data that you need. There is no real benefit to duplicating this info in another table, you just need this table to manage the relationship between the other two.
