Here are a couple of images on the best I have been able to do on a seam yet.
The two layers are laid face to face like normal then stitched with an unusual cam my old Elna's have that is 3 stitches one way then 3 back. This has worked OK because it does not crunch up the seam like a zig zag of the same width and it has a high level of stretch which is desirable with a high stretch fabric like this.
Because the fabric is a knit, it does not fray of a cut edge so you can get away with just trimming it reasonably close to the stitch edge line. The very light weight of the fabric ensures that it is no joy to trim as its like handling water but as long as you trim it reasonably accurately, you have a strong seam that maintains a high stretch level which is much the same size as an overlocked seam.
I have also done a similar seam using a 4 step wave stitch with a very short stitch length which works OK but the outside edge looks a bit notchy in comparison.