The "patch persist" as they call it is really exactly what I was looking for, allowing you to switch between any number of instruments and have their decays/sustains overlap -- glitch free. For anyone interested, here's a demonstration they have:

https://www.youtube.com/watch?v=y8t8p7JVy-0

It's not quite as polished as Cantabile, but seems more flexible overall.

Thanks again for all your information.

The software runs on both OSX and MS Windows.  You get it at https://www.gigperformer.com.

Probably for you, with a Windows environment, many MIDI routing solutions are possible —like the following:

1.)  Sequential Channel Changer, http://www.midikarval.net/OtherVST.html

2.)  MidiMapper, http://www.midikarval.net/MidiMapper.html

3.)  MidiMaster 2.0, https://www.kvraudio.com/product/midima...al/details

4.)  MidiStation 1.2, https://www.kvraudio.com/product/midist...al/details

5.)  MidiRouter, https://www.kvraudio.com/product/midiro...midikarval

Within just one site alone a number of solutions are compatible with your Behringer FCB1010!  That one site I saw is: http://www.midikarval.net.

Do let me know if you find anything, really, helpful.

Incidentally, I've a macOS software to program my FCB1010 from a computer desktop (that is) apart from the physical unit itself.

A similar software is available to Windows users: https://mountainutilities.eu/fcb1010.

It comes with how-to-videos:

https://www.youtube.com/watch?v=SYNND5nMWhc

https://youtu.be/MMJQ-AHRSMs

Until such a feature becomes a fixture, you’ll just have to assign to each of your loaded Pianoteq instances its own exclusive MIDI channel and select a new preset from your keyboard or foot controller only on some alternative MIDI channel to the one used by the instance intended to firstly sustain the long concurrent notes (chords)  —upon channel changes.  Presently you should both change presets and avoid note cut-offs, if you alternate Pianoteq instances through MIDI channel choices.  One instance could sustain its sound uninterrupted, even if you’ve recently switched to another, matter-of-factly, via any recent change in MIDI controller output channel selection!

The sustained notes coming out of one of two Pianoteq instances might remain unaffected by the other instance if you’re going to change to the other’s input channel from your keyboard or any other controller.  Which ultimately is to redirect MIDI to that instance that receives it purposely on an alternate channel.

Two Pianoteq instances, in combination, allow a total of four (4) preconfigured presets or patches (in A/B selection) to receive MIDI data dissimilarly on each respective Pianoteq instance's channel via the choices you make in output channel selection  —right on your MIDI controller.

Personally, if I were you I’d want to learn only as much as possible about MIDI and its terminology.  Thoroughly familiarize yourself with it, if you want to increase your proficiency at it and change your feeling badly about MIDI.  This personal advice is for anyone who considers MIDI overall important in his Pianoteq setup. 

Certainly one source of some truly valuable MIDI information is a book such as The Musician’s Guide to MIDI when it becomes available from Amazon: https://www.amazon.com/Musicians-Guide-...de+to+MID.

I'm assuming a reverb plugin could help somewhat, in that if I was not using Pianoteq's built-in reverb, then a separate reverb plugin would continue the reverberation sound while I switched Pianoteq presets. I've got some other reverb plugins I could use for that. Or does SPL Mo-Verb somehow allow you to sustain the sounds themselves (not just reverberant sounds)? What I would ideally like to do is play and hold a chord, then switch to a new instrument as the original chord continues to play, then once I play something else the new instrument kicks in.

I say that, as a last resort of course.  An alternative is probably this https://www.forum-pianoteq.com/viewtopic.php?id=5547.

Do you by chance know how to set a MIDI command to mute/unmute an instrument? I can probably figure out how to do it with a volume change if not.

Thanks again, I do appreciate it!

But, if you’re experiencing problems with two DAWs, I’d immediately say either you neglected to make a Gobal MIDI Mapping preset specifically for the FCB1010 or just neglected to load one whenever you opened Pianoteq in Reaper.

I had even less success with a different DAW, so I think for now I'll just have to simplify what I was attempting to do... running out of time for this performance.

Somewhat related feature request: I would love if Pianoteq allowed two instruments to be played at once, or better yet combined into a new instrument with both characteristics. This would be nice not only for the obvious reasons, but it might also eliminate the problem of having sound cut out when you switch to a different preset. For live performance the sound cutting out with sustained notes is difficult to work around.

If you’re on a Mac, you may use the Behringer FCB1010 with MainStage and macOS, of course.  Many professionals use it and MainStage together.