Just to clarify, they want to have the option to use either the PC or mobile client softphone, or both at the same time? The first option is a pretty standard setup. Just be sure to either have an external IP you can assign to the WAN side interface of MiCollab, or use a 2nd MBG in a DMZ to proxy the internet traffic. Then as long as you have a valid SSL cert (LetsEncrypt works too for budget-minded customers) and external DNS pointing correctly it's pretty easy and the user can switch which device has the softphone on the fly.
Once you have the server side stuff up, you just configure the users with a softphone extension, enable teleworker for it, deploy their MiCollab client and they can have multiple instances of it running on various devices (I have mine running on at least 3 different PC's and 2 mobile phones), but again, softphone can only be active on one at a time. You may be able to program more than 1 softphone in the user's MDUG, depending on licensing, but I've never actually had a use case for that.