It should not bunch. If you want to, some people adhere the two battings together with basting spray before they sandwich their quilt. Before I switched to Warm & Plush, I used to do a double layer of batting.
Any time you use a needle punched batting or batting with a scrim, you want to be sure to put both scrims facing the same direction (generally, that would be both facing down). If you don't spray baste them together (I never did), you want to be sure to carefully pin baste and check for any bubbles by touch in addition to looking at both sides of the basted quilt. If your machine allows you to reduce tension on your presser foot, I'd recommend doing so. And I would go with either a Microtex or Topstitch needle so it is super sharp to penetrate all 4 layers of the sandwich -- and the 6 layers when you're sewing on the binding to the back.