The snag with operating two boxes at once comes when there are simultaneous transmissions and you're not sure which one was for you, or even if either were - doing more harm than good. Most RADAR units will gladly let you call another agency and keep the assigned squawk but one unit I know say 'Squawk 7000, change en-route good day' then expect you to go through the whole diatribe of a new call to them 30 seconds later. That's still better than leaving the LARS frequency without telling them - a complete no-no.
We've one new 8.33 COM/NAV box and a second 25kHz box. We now only use the 2nd box for listening to ATIS etc. The new box has all the local frequencies programmed and is easy to flip between them. It also saves a lot of mucking about with our overly-complicated COM panel.
One tip - try and stay one step ahead of the next agency you need to contact sep up on the 'standby' side. You might not always be right, but be prepared. Also, write down every frequency you're given and read it back (along with pressure settings and squawk codes)
TOO