Sorry,
Yes, set top radio to COM1 and tune to tower / ground etc. Set bottom radio to COM2 and set it to ATTIS.
I believe there was a thread about even this not working properly for some aircraft such as the 152. Not sure if it has been fully resolved.
But only the active on COM 1 and 2 would ever be usable. Standby is there for east switchover / handoff.