One way would be to use the receivers capability to monitor a 2.88MHz wide frequency range at once, and while decoding one frequency it could continue scan in the background and monitor any other frequency that falls inside that 2.88MHz block and at least detect a carrier, and hopefully also if it is digital, and then when the monitored call ends it will now what next frequency in scan that are actually active with data.
Maybe that will need dual DSP if the CPU isn't powerful enough, but perhaps that 2.88MHz block can be output from the unused USB port to an external device (SDS500?) or a PC to perform that dual monitor function.
Or just get a $30 SDR dongle and monitor an important single local system using a PC and let the scanner monitor all other stuff.
Get a second dongle to also monitor another important system using the same PC or to scan the whole VHF-AIR in a couple of seconds that the scanner then do not need to monitor.
/Ubbe