Yes, you are still at 8ohm, but make really sure that the speakers you are bi-amping are indeed a true 8ohm load. You might not notice a big difference, depending on your receiver and speakers.
Just use common sense, and play safe (don't bi-amp, unless you are certain of your speakers 8ohm true impedance).
* I doubt that you'll notice a big difference, but it's worth a try.
** As for bi-wiring, I won't go there...