Added Dec 4, 2009
The twin needles create 2 parallel lines of topstitching and a zigzag stitch on the underside. The zigzag stitch helps maintain the stretchability in a knit hem. All the examples of twin needle stitching I could find showed a zigzag underside stitch where 2 different threads were visible. If your stitching looks like the above picture, you shouldn't need to adjust the tension.
If you notice sporadic bubbling in your topstitching, it probably isn't a tension problem (that would give you consistent bubbling). If you look at the zigzag underside of the fabric, you should see that your stitching is way past the raw edge of the hem in the areas of the bubbling. You just want the top row of stitches to catch as close as possible to the raw edge without going past it.