Simple Biamp question

I know there have been dozens of biamp threads, but my question is so simple that I'm sure someone can answer it instantaneously.
I have a preamp with 2 pairs of outputs and 2 identical stereo amps than cannot be bridged into mono status. So I wish to biamp my speakers (Allison 4s with biwire posts)
1. Can I vertically biamp with the left leads of both outputs going to one amp and the left speaker and vice versa with the right leads? I have heard from some Audio people that I cant do this. Why? Something to do with crossovers and imaging and soundstage....
2. Is it better to horizontally biamp witjh both amps sending single wires to lower frequencies and both amps sending single wires to upper frequencies?

I would prefer the verical, as it's neater and easier, but are therte drawbacks? Is there a reason both left and right outputs from the preamp cannot go into one amp and use it as a mono?

Amps : TAD 60
Prreamp: TAD 150 Signature

Thanks very much for your help.