@paul-flute
Haven't used Mainstage for a long time but I try to remember.
You want 2 ChordTrigger plugins, let's call them "Plugin 1" and "Plugin 2". You want to activate them alternatively. Both should never be on at the same time. But you want to turn them both off with the same control element on the Sylphyo.
The Sylphyo's "Bottom Edge Slider (CC#13)" looks applicable for this task, because it has a dedicated starting point at the very bottom. So it's not likely to operate it accidently. And this starting point (with a little hysteresis) can be used as a main switch, to bypass both plugins.
You want this function:
Sylphyo slider tapped, but very low: both plugins bypassed (this is your main off-switch).
Sylphyo slider a little higher until value 63: plugin 2 active.
Sylphyo slider over 63: plugin 2 bypassed, plugin 1 active.
Here is a screenshot of the two mappings:
It is quite simple, uses just an inverted range for Plugin 1. The plugin is bypassed from 0-63 and active from 64-127.
If you look at the graph symbol, you see that something was done in the mapping for Plugin 2. Here is the graph, that bypasses Plugin 2 if the slider value is 0-10.
(If the "stepped" view shows you different values, just ignore this Mainstage bug. The graph works.)
Here you have your function:
By default, both plugins are bypassed (set this "saved value" as attribute for both "Bypass" mappings). If you tap the Sylphyo slider at the low end (0-10), you activate your CC-13 but plugin 2 stays bypassed. Plugin 1 is off anyway because it needs a value over 63. Moving your finger higher then 10 activates Plugin 2. At the middle point Plugin 2 gets bypassed and Plugin 1 activated. This way you can switch between two chord settings. If you slide your finger fully down or simply lift your thumb, the Sylphyo slider sends a 0 and both plugins are off.
With the method above you have to keep your thumb on the slider as long as you play chords. I haven't thought about a toggle function yet.