I can imagine that it's still on the list, but I suppose the programmers never could spend the time needed, because they had other/more relevant things to do. Probably they were busy with the "next generation" mixers already, which at the given time were the RM rack mixers...

The RM mixers can be "cascaded" as "Master" and "Slave" and are seen as "one big mixer" in UC Surface, but the meaning of the words is slightly different and cascading is done via AVB network, not FireWire. The step of bringing cascaded AI-mixers in UC Surface together seems not to be really huge, but there are several of those kinds of little "gaps" in the two AI families. Some work never got done...

With UC Surface 2.0 it has got more difficult to select the AI mixers, because it only shows one rider on top. You need to go to the settings page to select the second mixer. Therefore an iPad in addition to US Surface on computer might be very helpful. It gives you direct access to the slave mixer and/or the auxes, just Smaart would be missing on iPad, which was planned for the coming firmware and software update, but got delayed again. It is also possible to add a computer screen to an iPad (special adapter), so if you want the visualization located next to your computer screen, this is a nice "workaround".

PreSonus support gave you the correct answer and this is just a users' forum. But thanks for asking and reminding of the forgotten list of promised features...