You have a few options.

Foot pedals to "chord" the keys to send multiple control, alt combinations (you can now do left alt, and right alt as two separate modifier keys as shift and control too). So to chord the keys, combine left alt + wsad for example.

You could have a button to reassign focuses to the new character such that you can change focuses on the fly and it would not matter who you used as your "main". That same button could dynamically update keymaps, but you would likely need a UI mod to do so - or keyclone to support that. This would work best for 4 man shammys for example, when it does not matter who heals or who DPSes and you have all 4 screens visible. It of course could be used to swap out to a more expendable character to charge into arenas, etc.

You can have a dedicated Xkeys setup to control movement for your characters. The top row of a PRO is suitable for this for 4 characters. 8 or more if you combine with chording.

There are other options out there too but the change of focus seems to be one that is easy to implement and I suggest designing your initial macros with this in mind. It is a bit of a pain to change them later.