Well, one body would be visible, with some mass. The other would not be visible, with some mass.
The current problem is that the inertia is being calculated wrong as it assumes that the mass is evenly distributed, which it is not.
That's why you are currently doing a wheelie.
So, if you actually calculated the form of inertia that you want, then you could just use that instead of complicating the simulation.
I don't really think that I can do that.. I am still new at this.. also about the inertia thing you said, I found a function SetMassProps which you can set the mass and the inertia, but I am not sure what I can do with it yet...