I never thought of the bobbin thread. Now I know not to try a double needle, thanks.

the only time i've used the double needle was to sew both sides at the same time of a stem, or such, for applique.
i didn't like it that much, so haven't done it again

I can't imagine something different because you only have one bobbin. I tired it once in the early 90s and did not like the zig zag created by the bobbin either.

video of how to thread your machine

https://search.yahoo.com/yhs/search;_ylc=X1MDMjE0MjQ3ODk0OARfcgMyBGZyA3lmcC10LT kwMARmcjIDc2EtZ3AEZ3ByaWQDBG5fZ3BzAzEwBG9yaWdpbgNh dHQueWFob28uY29tBHBvcwMyBHBxc3RyAwRxdWVyeQNob3cgdG 8gdGhyZWFkIGEgZG91YmxlIG5lZWRsZSBvbiBhIHNld2luZyBt YWNoaW5lBHNhYwMxBHNhbwMx?p=how%20to%20thread%20a%2 0double%20needle%20on%20a%20sewing%20machine&fr=yf p-t-900-s&fr2=p%3Afp%2Cm%3Asb&hspart=att&hsimp=yhs-att_001&type=att_pc_homerun_portal

I've used a twin needle to quilt, not caring that the back would have a zig-zag ... but it WILL. Without a double bobbin (does any machine have a double bobbin?) you can't get away from the zig-zag on the reverse.

The craftsy site doesn't have a good picture of the reverse. You can only see a single line of stitching.

This video shows you what the reverse looks like.
http://www.howcast.com/videos/512586...ewing-Machine/

If there are 2 threads on the top and 1 thread on the bottom the bottom thread has to do double work.

I use the double needle for perfect top stitch quilting. I only thread one needle and use the unthreaded one to sew in the seam. This way I get perfect spacing from the seam to the stitch.

Just today I tried a double needle for the first time, to hem a pair of men's casual slacks, and it worked beautifully because the underside won't be seen. I'll try it again in quilting my bowl cozies (microwave bowl holders) because I quilt each side separately before assembling. It could work well in a bag or tote that will have a separate lining. So yes, the underside is ugly, but if that won't show, try it, because the double needle makes a perfectly aligned double line that can rarely be matched by sewing a single line twice. But don't try going around any curves unless you're wanting puckers!

Onebyone, I just read your post about using a double needle for echo stitching with only one needle threaded, and I will definitely try your brilliant idea!

I learned this in my 9th grade Home Ec class. We used it for perfect top stitching.

I think the older home ec classes in high school was the equivalent of several years in professional design sewing schools now.

One of the reasons for using a double needle is to get the zigzag on the back thus allowing for a bit of stretch. This is great for the hem on a T-shirt or other areas that you would like a bit of stretch and don't want to use a serger.
If you use 2 threads on the top and 1 thread on the bottom, there is no way to not have the zigzag - it's how it works. If you want the look of the double stitching on top with 2 straight lines on bottom, you will have to stitch the seam and then echo stitch a second line.