Apologies, I dont know enough about xml or I could have worked this out:
Can a combination of regions be the trigger for a conditional border?
The instinctive answer is no, but I am wondering if the combination required could be set-up as a continent with no bonus? So, the two regions are just considered a continent in the code so that the continent name can be used for the conditional border code. However, if this were possible, would the log read something daft such as 'Player X receives 0 troops for holding ...' ?