manicmike

Your expectation is correct. It *should* look like a nice even zig-zag. I'm going on my experience of double needles on actual (50+ YO) vintage machines though. Maybe modern or stretch make some difference but it still doesn't make any sense for it to be skewed.

I'd think the needles must have different upper thread tensions to exhibit this problem. Since both threads are the same weight and go through the same tension discs, the culprit is probably the tension mechanism. To check, remove the threads from just the needles (so you don't bend or break them when you pull), put the presser down (to engage the tension) and pull the end of each thread in turn. One should be obviously stiffer. You will have one that's either too loose or too tight. Can't tell you which, but I suspect tight (if one was very loose, the lower stitch might include loops). Either way, if one is tighter I'd suspect dirt on a tension disc. The solution is to disassemble, clean then reassemble.

Perform the thread pull test again and you should find that both have the same tension. Thread it up and do a test sew.

I'm really making a lot of assumptions there, but the diagnosis is right you'll want to take photos during disassembly. Very easy to forget how it goes together.