One of the reasons for using a double needle is to get the zigzag on the back thus allowing for a bit of stretch. This is great for the hem on a T-shirt or other areas that you would like a bit of stretch and don't want to use a serger.
If you use 2 threads on the top and 1 thread on the bottom, there is no way to not have the zigzag - it's how it works. If you want the look of the double stitching on top with 2 straight lines on bottom, you will have to stitch the seam and then echo stitch a second line.