I have an XJ-SC targa convertible with a rear softtop. I have also seen XJ-SC targa convertibles with a rear hardtop. Does the hardtop fit ontop of the open softtop? Or is it an either one or the other? Would I have to remove the entire softtop from the car to install the hardtop?

I love having problems like this. :-)

 

Yes, you would have to remove then to install.
Very difficult. Four hands are needed.