The second AHB2 will give you more volume on your speakers. On my NS5000 speaker my stereo AHB2 is about 90% of max volume for about 70-80 dB. I like to use the lowest gain setting on the amp. The middle gain setting does get you a bit more granularity on the volume, but it is not as much as I want. It also varies by sources, SACD, tuner, and DAC. Having monos eliminates this important issue for me (I had monos before).
I am debating whether to trade my silver AHB2 for a HPA4 that someone is looking to trade on USAM. I am not going to use the AHB2 anymore and will stick with the CODA amps for my LRS+ and the NS5000. I expected the AHB2 to be great with the NS5000 but it was not as good as the CODA. Though I bet one day I will say that I wish I had an AHB2 around to try something.
If I had my shipping box for my Holo Serene preamp I would jump on the HPA4. Debating now. I use the LA4 on my Livingroom.