I think both are active. As I have set tower on COM1 and Ground on COM2, I can hear both transmissions.

Alternatively you can edit the panel.cfg and add the default COM-Group, it should look like this:

[Window Titles]
Window00=Hauptinstrumentenbrett
Window01=GPS
Window02=Funkgruppe
...
Window08=Com2 (you can set the name as you like)


//--------------------------------------------------------
[Window08]
BACKGROUND_COLOR=2,2,2
size_mm=250,556
position=8
visible=0
ident=RADIO_STACK_PANEL

gauge00=Bendix_King_Radio!Bendix-King Radio Audio,      0, 0,  250, 50
gauge01=Bendix_King_Radio!Bendix-King Radio Nav-Comm 1, 0, 50, 250, 94
gauge02=Bendix_King_Radio!Bendix-King Radio Nav-Comm 2, 0, 144,250, 94
gauge03=Bendix_King_Radio!Bendix-King Radio ADF,      0, 237,250, 82
gauge04=Bendix_King_Radio!Bendix-King Radio DME,      0, 319,250, 82
gauge05=Bendix_King_Radio!Bendix-King Radio Xpndr,      0, 401,250, 78
gauge06=Bendix_King_Radio!Bendix-King Radio AP,      0, 479,250, 77

The Default-Coms keeps untouched, you have only set the right Window-Nr. Keep in mind that the Hotkeys (SHIFT 1-9) only works to Window08 all other panels you have to select from the menu.