Congrats on the PMC-10, probably the ideal MIDI footswitch for the Triaxis and Mesa 2:90 with MIDI effect units.
I won't give you a complete step-by-step, but here is the guist of it. First set the Triaxis to respond on a different MIDI channel than your Quadraverb, for example, Triaxis on Ch 1 and QVerb on Ch 2.
Select a bank and button (1 - 9) on the PMC-10, and assign a patch to the button, for example patch number 60 assigned to button #1 in your bank. You can name the bank something like "Jazz," "Heavy Rock, "
Blues" or whatever.
Edit the patch assigned to the button #1. You can name it after a song, for example "Red House" or a sound "Basic Clean" etc. Then, set up the patch to send a Program Change command on Ch1 to control the Triaxis. For example Program Change 28 will change the Triaxis to preset 28. Make sure that preset 28 on the Triaxis has all the settings that you want, including 2:90 switch settings and EFX loop on off.
If you want to combine this Triaxis preset with a Qverb preset, program the PMC-10 patch by adding a Program Change on Ch 2 to it. Selecting button #1 on the PMC-10 will then change the Triaxis to preset #28 and the QVerb to whatever you selected.
To turn the EFX on and off, you can copy the PMC-10 patch (60 in the example) to the next empty patch, probably 61. Assign 61 to button #2 in your PMC-10 bank. Then edit the new patch to be identical to 60, except that you now send a different Program Change to the Qverb. You can for example set the Qverb to a noise reduction or bypass mode if it can do this. If not, then you will have to clone the Triaxis preset to an identical one with a different number with the EFX loop off. This can use up Triaxis presets. You have what, 99 available?
If you set the PMC-10 in Enhanced button mode, then after you go from button #1 (effect on) to button #2 (effect off, bypassed or simple noise reduction), you can switch from one to the other just by hitting the same button over and over to toggle from the present preset to the previous one.
This can all get complicated, so the best thing to do is create lists of patches and presets for the QVerb, Triaxis and PMC-10 on three separate pieces of paper. After you have put in some programming time, you should also hook each up to your PC and do a SYSEX dump to back up all of the presets. If something gets nuked, you can restore from the PC.
If you arrange all the Triaxis presets in some sort of logical grouping, if all of your MIDI stuff goes out to lunch during a gig, you can still find your basic tones by pushing the front panel buttons.
For tips on the PMC-10, look for the user group on Yahoo.