The axles will need to be supported in two places. You could say that the axle IS supported in two places already... the inner and outer bearings in the toughbox, but they are pretty close together. If you keep the wheel really, really close to the toughbox, though... you'll probably be okay.
It is an issue of torque... when using just the toughbox bearings, the axle is a lever and the bearing nearest the wheel is the fulcrum. As the wheel gets further away from the fulcrum, it starts to develop mechanical advantage and pushes "up" on the bearing near the wheel, and "down" on the bearing away from the wheel.
The toughboxes are pretty tough, but I'd still consider putting some outboard bearing support in there. You'll probably be fine with the wheels set up as you plan, but I can guarantee you'll be fine with the wheels set up like this:
http://www.youtube.com/dtengineering#p/u/0/xgTJcm9EVnE
Jason