D9,
If it is not prohibited in the Type Certificate or Flight Manual and you are within CG limits, then you can. However, I think you should clear it with your insurance company first and be prepared for some potentially distinctly different handling characteristics, due to the CG being way aft.
I know of some two-seat gliders where it is theoretically possible (with perhaps a few pounds of ballast in the front seat), but is prohibited, usually because some controls are missing from the rear seat. For example, in the case of the Blanik (RIP), there was no rear canopy-jettison lever. In the case of the DG-1000, it is just prohibited with no reason given.