Hi becki
I’m in a similar situation and as far as I understand you need separate wordpress multisite installs, one for each domain, to get this working.
The Network setup in the Multisite install requires to choose between one of two options: sub-domains or sub-directories. As you don’t want to use the language as part of a sub domain it’s clear, you have to install one multisite install for each domain.
There might be tweaks around but they are certainly not supported.
