This question has been asked in various forms on many occasions, including by me. I'm no expert in this regard, but I did stay at a Holiday Inn Express once, so I'll take a shot with what I think I know. The Skyview will display frequencies for two com radios, but only one at a time. You can only control or push (TUNE COM) the primary com radio from the Skyview EFIS. This can be a Dynon or other compatible radio, the SL30 or TY91 for example. Using a Dynon radio also gives you the ability to transfer database frequencies using the TWR, GND, ATIS and ATC buttons, but that only works with Dynon radios. For the secondary com radio, the Skyview only gives the ability to display the frequency. The secondary radio cannot be tuned from the Skyview, no matter who makes it. It must be tuned using the radio controls themselves. I'm not certain about the ability to push database frequencies to a secondary com radio from the Skyview, but I believe you can as long as it's a Dynon radio. As I understand it, you cannot push database frequencies to a non-Dynon secondary com radio from the Skyview (someone correct me if I'm wrong). Although Dynon radios are repackaged hardware from Trig Avionics, they use different protocols, and will not have the same functionality as a Dynon radio (someone also correct me if I'm wrong there). Someone else would have to tell you what goes via bus as opposed to serial port. I don't know enough about that to attempt an answer.
I'm kind of hoping someone more knowledgeable will validate and/or correct all of this, because I've had some trouble wrapping my head completely around it for quite some time now.