by that I assume you mean a wagon... just for clarification: Trailer = one axle. Tandem Trailer = two axles close together, toward the back of the bed and neither steer. Wagon = two axles, front axle is steerable and at the front of the bed and the rear is not steerable and is at the back of the bed.
On another note, why couldn't you fab the tongue so that it would not move side to side (but still able to swing vertically) and then for front wheels use zero turn mower style wheels. That way they could be fully load bearing and the "wagon" would still back like a "trailer".
by the way, sorry for the thread hijack...
That is a Very interesting Idea!