@Mickey2: Thank you for the tip on using quilt fabric! It got me thinking about what fabric I was using in relation to the needle. I was using t-shirt type fabric with a 14/90 needle. At best I was able to get both threads showing through at the same time on both sides of the fabric. I found mention online that this can be caused by the needle being too big for the fabric. I switched to a fabric about twice the thickness ("Cozy Flannel"). Stitches were coming out a little better. I doubled the flannel layers to 4 and got the stitches looking correct on both sides. Pulling the layers apart, the two threads appear to be crossing in the middle, and neither stitch is showing through on the wrong side. I will be using the machine to sew through heavy faux-fur fabrics, so I am not it any rush to get thinner needles.
P.S. I noticed the PDF doc linked to on page 1 shows a different tensioner set up than my 153, with different instructions.