Vertical bi-amping relates to using two amp channels to drive a single speaker (specifically, using one two-channel amp to drive the high and low frequency drivers of one speaker and a second two-channel amp to drive the high and low-frequency drivers of a second speaker). We're not really dealing with bi-amping of any sort here (we're not using two amp channels to drive a single speakers, but using one amp channel to drive multiple speakers). We are simply splitting the left and right pre-amp outputs in order to drive multiple amps - all it takes is a pair of Y-adapters, which should not be a detrimental arrangement if you use a reasonably decent Y-adapter. Of course, if you use a single amp and a larger distribution system downstream of it (such as the 8-speaker distribution block I found earlier today) or a single amp with two sets of binding posts that can be used simultaneously, you wouldn't need the Y adapter.

The 990's second zone is an analog pre-amp. By that, I mean that digital sources are not routed to the second zone output. You would need to make stereo analog connections from sources that you want access to in the second zone. The analog section itself is good - you'll also be counting on the analog output of the source devices (CD or DVD player, etc.), and of course the amp and speakers. If it were me, I'd use the 990's second zone to drive a system like this.