The tape used is called Heat Press Batting Together and is available at almost all quilting stores and online sites...Connecting Threads has it.
I have been taking the Craftsy classes by Ann Peterson. She has two coarses "Quilting Big Projects on a Small Machine" and "Beyond Basic Machine Quilting". She is such a good teacher so easy to understand and like our Jenny she likes to use things around the house rather than buy something. When joining pieces of batting together, she suggests to cut it in a large wave cut. Then either baste it or Heat Press Tape it together. She shows this in the split binding session for FMQ a large quilt in pieces.