The 'multiple domain' approach was not really designed from the bottom up; it sort of evolved over time as a way to keep track of various things in the P&H.
At any rate; a regent cannot oversee more than one court at a time. So if you have more than one court you are effectively not running a combination domain, but must appoint separate regents as vassals. Then THEY can have their won courts.
So if GeM wants more actions and more flexibility she could split her domain into two parts, with say a former lieutenant invested as the vassal regent of the PCE, while she keeps her sources and Ilien.