As near as I can gather from what I've seen & heard, the second antenna is a 'steerable' high gain, narrow beam antenna. Also, as near as I can gather, it's totally compatible with the existing ePMP SM's, so that we can keep deploying ePMP1000/200/180's and the new AP's will be compatible with them.
So, what does this do? Well, as near as I can gather from that video, since the Cambium AP's already tell each SM when they'll get their time-slot to talk, I gather that they steer the antenna to listed in exactly that direction, at exactly that time. That would SIGNIFICANTLY increase the upstream RSSI and significantly degrease the noise heard by the AP. The extra gain may also allow the SM's to be able to APC their transmitters down even more and reduce interference on other sectors as a result. If I'm right about this, then this is all a really really good thing - since it's all about SNR.
That being said - I agree that it would be nice to have them both built into the same antenna enclosure. Actually, I'd love a combination 2.4Ghz & 5Ghz antenna as well - we could more easily deploy both freqs on all our towers.