You'll find
this post by the noted designer Steve McCormack to be well worth reading.
Also, I believe that contrary to widespread belief the 3db power increase (i.e., twice the power) that is commonly cited for bi-amping vs. single amping will in many and probably most cases not be realized. In the common situation where power delivery of the single-amp arrangement is limited by voltage swing capability (i.e., by clipping, as opposed to being limited by the amp's output current capability), the power increase provided by bi-amping will be 0 or close to it.
For equal volume to be produced in the bi-amp'd configuration vs. the single-amp'd condition, the two bi-amp'd amps will both have to output the same voltage swing as a single amp would have to. So the volume level at which the clipping point will be reached will not be changed by bi-amping, other than to the extent that the available voltage swing capability of the particular amp design may vary as a function of current demand (although that may be significant in many cases).
What will be helped is that the amount of current each amp channel has to supply will be reduced, which may improve the sonic performance of the amp even if it would not enter current limited conditions in the single-amp configuration. Also, vertical biamping will provide the benefits Steve describes in the post I linked to, eliminating crosstalk across the common power supply and ground system that can occur to some degree with some amps in single-amp or horizontally bi-amped configurations, since in those configurations the two amp channels handle different signals.
Regards,
-- Al