Originally Posted by Tartan
I use my rotary cutter on the two batting edges to get them perfectly straight. Then I machine zigzag them together. Works great.
Ditto!
You need to make sure the two line up perfectly straight and flat ... or after joining, you'll have bumps and lumps ... which will affect your final result.
And definitely, do not overlap, or again, bumps and lumps you don't want to deal with.