I still have no idea how to simultaneously control more than one parameter from a single control, but I did manage to solve my particular problem with emulating the Multi Delay time/sync knob.
My mistake was thinking that a stack group could only be controlled by a Variable. I guess because that was mainly how the examples in the manual were structured, it didn’t occur to me that I could use an H6 parameter to change the stack status instead. So, in case anyone else is interested in doing this, here’s the procedure:
Make a switch and assign the temposync1 Multi Delay parameter to the switch value. Name the switch Sync.
Make a knob and assign the delaytimeoverall Multi Delay parameter to the knob value. Name the knob Time.
Duplicate the knob and position it directly behind the Time knob. Assign the syncnote1 Multi Delay parameter to the knob value. Name the knob Sync Note. You’ll see both labels overlapping.
Make a Stack in the GUI tree. Assign the temposync1 Multi Delay parameter to the stack value.
Put the delay Time knob and the Sync Note knob inside of the new stack. Make sure that the Rate knob is above the Sync Note knob in the GUI tree. This is what makes the Time knob appear while the Sync switch is off and the Sync Note knob appear when the Sync switch is on.
Now, when you activate the Sync switch, the knobs will switch out and time will be controlled when the switch is off and sync note will be controlled when the switch is on, just as it works in the Multi Delay interface. This trick also works with any of the other effects that have a sync switch - for instance, the modulation effects.
Disclaimer: I’m a complete newbie at this, so I won’t be surprised if there’s a more elegant solution, but I just wanted to throw a method out there that works in case someone else wants to do something similar.