You can try both ways...one may sound better. Conventional wisdom says vertical bi-amp...#1
I biamp my Linn speakers using horizontal biamping, which is what Linn recommend. For Vandersteen, I would give great respect to what Mr. Vandersteen says, as noted in the post/link above.
For your application try both and see which you like. There are (or should be) some differences.
Although you have identical amps, horizontal biamping allows you to try some interesting things by using different amps. For example, you could put a more powerful amp on the bottom for greater woofer control. Or you could try mixing tubes and solid state for top and bottom. Just make sure the gain is the same if you experiment in the future using different amps.
Thks to all...I skimmed the Vandy FAQ and the most applicable Q seems to be in reference to trying this with a multi-channel HT-type amp which Mr. V discourages and suggests more suitably matched 2-ch. units...I think the NADs are more than capable in this app...will just take some experimentation to find the right combo as everyone suggested...glad also to learn the "horizontal" and "vertical" nomenclature!!!...I may try to repost the question on the vandy site, but will more than likely just experiment a bit and draw my own conclusions...after all...it is the trying out of different combinations and configurations that draws me to this hobby...almost as much as finding a satisfying result!!!
When I tried biamping for the first time, I wanted to use Vertical. But, after I switched to experiment with horizontal biapming, I never went back. I had zero separation with vertical biamping, and all the separation I wanted with horizontal.
I'm surprised that no one has brought up the idea of an active crossover yet.
I toyed with the idea of active CO and was seriously considering trying my hand at Linkwitz's DIY Orion speaker which is fully biamped (triamped???)and requires the builder to also construct an active CO/eq of Mr. Linkwitz's design...each side requires FOUR identical channels of amplification...however, that project requires one to commit a couple grand just to get started and is pretty involved technically...I still may at some point but want to explore the biamp concept in a decent passive system first...I have minimized my outlay with decent used gear...if I like the concept, I will likely segway (sic) from here...perhaps to one of the pro-sound actives that can be had on the cheap...
for any who are unfamiliar with the Orion, here is the link:
An amazing site and an amazing speaker (many rate it among the best available at any price) designed by a legendary figure in sound reproduction...