Bi-Amp Basics - NHT 29s

Depending on what connection options your amps have, you may also be able to daisy chain the amps so that you don't need a splitter.

Keep in mind that when bi-amping, the amps need to have the same gain

What you might want to consider, using the five channel amp to drive the 2.9's and the AC2 and your current amp to power your surrounds. Use four of the channel to vertically drive your 2.9's and the fifth to drive the AC2. This would make for a more consistent front soundstage. Then when possible find a matching two channel version of the five channel amp. Another option 2 Two channel amps to vertically bi-amp the 2.9's and a three channel amp to power the surrounds. I've found the McCormacks to work well with the NHT's. I have a pair of the 2.5's connected to a DNA-1 in my spare room. If you decide to use the older McCormack's DNA's I would recommend the DNA-1's. Heard the 2.9's with the DNA 0.5's and it was really bass shy.

Another Option
Use two Aragon 8008X3's and pick up a NHT SW3 Slave and you would have one awesome 5.1 and 2 channel system. The Aragon's work great with the NHT's also. Not sure if these are within your budget. But just a suggestion.

Get a NHT X2 active crossover and a pair of NHT A1 monoblock amps to bi-amp the 2.9's subs. They are much better than the older SA-2/SA-3 amp/crossover combos. This will also give you more control over the bass by allowing you to dial in flat response for music, and at the same time, a little extra "boom" for movie soundtracks' LFE content. Then, for the 2.9's upper frequncy drivers, since they are high passed at 100 Hz, power rating simply isn't as critical. Go for quality over quantity.

Also, replace those dipole HDP2 surrounds with a pair of new Evolution L5 on wall surrounds on closeout from ListenUp.com, which are phase-matched to the 2.9s/AC-2.

If you do these things, I guarantee you that you will be happy with your system for many more years to come.

Follow Markruss' advice of using an active crossover to bi-amp. Power amps w/o active filtering MUST have matched gain AND sensitivity to provide an accurate frequency response when used to bi-amp(or at least input attenuators). An active crossover will give you the ability/flexibility to balance whatever power amps you choose with one another.