Nice to see this topic about idler wheels, I also use them and my drive train performs very well. I can go up insanely steep hills without slip using a 12T (too small) motor pulley and minimal belt tension. As soon as I change to a 15T motor pulley I think it would do even better and need less frequent belt adjustment.
I used two idler wheels, which looks great, but honestly I dont think it is much better (if at all) than a single idler.
I face the same problem that on a normal size motor mount, the wheel is in the way such that you cant place the idler bearings, especially when you use 97mm wheels like me there is zero space. So what I did is I took two normal shoulder bolts, put them in the lathe, and cut off the head until it is only 0.5mm. I then used a saw to make a slot in the other (threaded) side for using a flat head screw driver. This way the idler bearings can sit between the wheels and the motor mounts as you can see in the picture.