I'll phrase it like this: I want effect 1 to turn off when I engage effect 2. And both effects come from different blocks.
I get your point: I would want my delay to be on w/ the Plex in some instances. Or were you just asking to clarify?
A few ways to do this -
Multiplexor is a cool idea, have the effects in parallel. I would then have a shunt in parallel too (so three lines into the Multiplexor) you then use the switch to move through the channels on the multiplexor. First hit is delay, second is plex, third is both bypassed. OR maybe press switches between the two delays and hold moves it to both bypassed. That's a slick solution
Scenes - have two scenes the same but with that delay difference, then the switch just moves between scenes, problem here is if you want to choose other effects on the fly that doesn't really work.
But both of the above doesn't work if you want both on at once, that's the challenge.
It seems to me you're missing up use cases - if you want them to alternate but sometimes not? That's hard - two ideas there:
The footswitch turns off and on the delay (so it's a straight effect bypass), and you use an expression pedal to control the mix of the plexdelay, so you can fade it in over the top.
You have a mixer instead of the multiplexor - and connect as I described with the multiplexor. You then have four channels of the mixer, channel one is just delay, channel two is plexdelay, channel three is bypassed signal and channel four is a mix (so both on) and your pedal moves through those channels. You could then attach an expression pedal and use it to control the mix/blend if you wanted to as well.
Last idea:
Have the delay so it's bypass state is controlled by a control switch (cs-1)
Have the plexdelay so it's bypass state is controlled by the same control switch (cs-1) but 'flipped' so when the switch is on the effect is off. Have the second channel of the plexdelay so it's also controlled by the cs-1 but in the traditional sense (i.e. the same polarity as the delay).
Your footswitch then toggles the cs state, meaning you can flip between plex-delay and delay. You then set the hold function to change the channel of the plex-delay meaning if you hold the switch down it will change the channel on the plex and it will also become active.
All these are from the top of my head, but I reckon they'll work