So here's the whole story: There are some things about SkyView Classic/Touch's software architecture that make it harder to develop on against vs HDX. So, we made an early decision - much like we did with SkyView vs the earlier D10/D100 series, to treat SkyView HDX as its own product so that it's not encumbered by future compatibility issues. This also helps keep the robust set of testing that we perform as we develop and release software in a reasonable place.
Behind the scenes, the moment that HDX shipped, the codebases were so similar that everything basically worked. It may even be the case today, but we're not tracking it (it's not a supported configuration). BUT, and this is very important: we don't do any compatibility testing between the systems (again, it's not a supported configuration). If there are compatibility bugs that crop up when an HDX is connected to a Touch/Classic. One more thing - even though things may APPEAR to work well, there are a lot of behind the scenes things going on, such as configuration settings syncing between displays, that may not be obvious immediately when they don't work right, but which can cause unexpected behavior.