Hi

The OPS Manual says::

NOTE: DUE TO TECHNICAL LIMITS IN THE PMDG 747, FOR COMM 2, YOU CAN ONLY SET STANDBY COMM 2. YOU MUST THEN USE THE SWAP COMMAND TO MAKE THE FREQUENCE ACTIVE.

As an alternative procedure that allows you to set both COM2 and COM2 Standby using the normal commands the following can be done:
On the Captain's side:
1. Select COM 2 Receive - not MIC (COM 1 MIC and Receive can be left as is). This then allows the normal commands to work on both COM 2 and COM2 Standby. 
2. Deselect COM 2 Receive on completing changes to the frequencies.

Hope this of some help.

Ray

 

 

 

Thank you for sharing!  That's an interesting technical point