Here is a link to the section of the website that includes serger reviews:
http://sewing.patternreview.com/SewingMachine/Reviews/
I have used a double needle in my Bernina sewing machine. You thread the machine with two threads and use a straight stitch. The top looks like parallel lines of straight stitch; the underside looks like a zigzag. Basically the single bobbin thread zigs and zags between the two top threads to form the stitches. When using a double needle on stretch knit fabrics, I found it very easy to stretch the fabric out of shape while sewing because the sewing machine doesn't have the differential feed option. Sometimes I would get tunneling also, probably from not having the bobbin tension loose enough for the fabric. Sergers do many more stitches per minute than sewing machines also, so sewing a hem with the serger should (theoretically, anyway) go faster than on a sewing machine.