In TIM the monomers are hardly involved in each others activity, just one residue
that helps holding in place one active site residue in the other monomer.
The two monomers in the HIV protease are 100% equal partners in the process.
One can imagine making an active TIM-monomer (and indeed that has been done
by extensive bioinformatics-supported protein engineering), but with the HIV protease that
will be impossible.