It worked very well.
I set up both cone stands, one with white and one with red serger thread as the top thread, and the yellow Coats machine quilting thread in the bobbin:
I grabbed a small scrap of denim and went around in circles. Worked really good.
Then tried it on pre-quilted scraps:
If you look really close you can see the yellow bobbin thread towards the top and the red and white top thread at the bottom. The tension never gave me the slightest bit of trouble.
Then I grabbed a bigger piece of denim and doubled it:
It sewed through that just as it would have with a single thread. No problems at all.
To get both threads through the needle I had to go from a size 14 to a size 16. I couldn't get both threads through the 14.
I also noticed that as the two threads entered the spring of my widget they twisted and formed a single thread. I'm wondering if it is the circles of the spring that created that effect. No matter how it happened it worked very will through the tensioners, guides and needle.
My next experiment will be three colors of variegated thread. That should be pretty.