Can I bi-amp my speakers with a Denon 3802?

Only if you want to hear the main front to the one part of the speaker (the woofer for instance) and the surrounds or backs speaker outs to the the tweeter/mid section. It will not work, you just can't assign what signal will go to which discrete amp, the digital surround processor will encode (5/6ch stereo, DD, DTS etc.) and assign a signal to the amp, the ones you have enable thru the speaker menu.
I hope this helps.
Regards,
Rich

Thanks guys. I didn't think it would be feasible. I'll just bi-wire them instead.

Passive bi-amping IMO, is not worth the effort.
You will not increase the dynamic range or prevent low frequency signals from impacting the high frequency response, as in active bi-amping.
One of the main benefits(maybe THE main benefit) of active bi-amping is the lack of a lossy, passive crossover network.

Adding a powered subwoofer and high-passing the mains would be closer to a true bi-amped system, and would absolutely dwarf passive bi-amping in its effectiveness at increasing the dynamic range and clarity of your system.

If you vertical bi-amp and biwire then you won;t need to add an external active crossover.