Bluntly put: Scanning between two or more trunked systems, or between trunked and analog, is highly inadvisable.
I'm not familiar with the operation of the Harris radios, so I can only speak with certainty about the Motorola radios.
The APX series will allow you to scan across multiple trunked systems, as well as mixed trunked/analog scan. This is known as Multi-System Talkgroup scan, and while the name implies that it's only for trunked systems, you can in fact mix trunking and analog in the same scan list. Note that with this type of scan, you cannot set any type of priority channel.
However...just because you can doesn't mean you should!
The advice the Orange County coordinator is giving is valid and sound, not BS.
In order for the radio to scan between trunked and analog, it must be aware of the control channel. Scanning involves switching between decoding the trunked system control channel for X amount of time (looking for talkgroup grants), then leaving the control channel to scan the analog frequencies, and so on. This results in what amounts to a very inefficient scan operation that will inherently have some delay when switching between trunked and analog scan list members. The same is true if you're scanning two trunked systems even without any analog channels mixed in; the radio must still bounce between each system's control channel to look for talkgroup grants.
And once again, remember that there is no priority available with this type of scan, so whatever active talkgroup or analog frequency the radio lands on, that's what you're going to listen to until the transmission ends and it resumes scanning...regardless of what else is going on.
So aside from the technical reasons why it's a lousy idea to scan across systems and/or with a mix or trunking and analog, it's also bad from an operational standpoint. There is no way you can effectively monitor communications on two different resources using a single radio...at some point, something critical is going to be missed.