I have a very old Kenmore and it has two vertical thread holders on the top.
It has zig-zag capability and can accommodate as much as a 4 mm twin needle.
The two threads do fine going through all the same guides until right before
the needle-threading step. My machine has 2 metal loops just above the needle.
I use the twin needle primarily for stained glass quilt blocks. The 4 mm is perfectly sized to sew the 1/4" bias fabric that serves as the "leading" in the
stained glass.
Just experiment and you will find out whether your machine and you can work
with the 2 threads. It's a nice technique to have in your sewing armamentarium, but no need to stress about it. What's the worse that can happen - if the needle breaks, you can buy others. If you get a big knot
I'll bet that's happened before. Be playful. - Judy