on Airbus FBW: by using the select function to tune the offside VHF; the previously active frequency of that box is lost, so going back to the previous frequency is not possible,
Not correct. The previous frequency is displayed in the 2nd window of the RNP used to change that frequency. All that is required to go back to the previous frequency is to simply transfer the frequency using the same RNP again. However you have to remember who changed the frequency last.