On every OBD-II car I've seen the downstream sensors were there primarily for checking for the existence of the pre-cats. The two upstream O2's are the ones that monitor the air/fuel ratio. If the voltage drop between the the up and downstream sensors was within spec, then no errors were thrown. If the voltages were too close (or the same) then something was amiss with either pre-cat and needed fixing.

Now, this doesn't necessarily mean that you can just do away with the downstream sensors and call it a day. The ECU will still be looking for them to be installed and will still be wanting to see proper voltages. Therefore, while the primary function of the downstream sensors is just to monitor catalytic efficiency, if the voltages are out of spec, the ECU might complain and cause the car to dump fuel (run rich) to prevent any kind of engine damage.

If you would like to provide more information about your project I might be able to tell you more about where you could potentially run into trouble.