I also use a twin needle 90% of the time. I have a coverstitch feature on my serger but do not care for the set up involved. I know I need to buy a separate one, but LOL where would I put it.

The twin needles will bunch up the fabric (inbetween) if you have the tension too high. I keep my stitch length at 2, and just loosen the tension, oh and cross my fingers.....

mrs slater
(sorry if pic comes out huge)