On our 9 mm machines with the zig zag plate on it is possible to do twin needle embroidery but you do need to be careful. As Janome embroidery machines stitch during embroidery in the left of center position, you must make sure that the double needle does not hit on the left side of the plate. Try the double needle that came with your machine.
If you have a 7 mm machine, then there used to be a special twin needle available but I have not seen it for years. Without the special twin needle, you cannot to twin needle embroideries.