This reply is going to get a bit convoluted. So bear with me.
If you're monitoring a handful of known frequencies there are many options to choose from. Searching the entire air band - especially the UHF Mil allocation - gets into some specialized hardware, software and techniques.
There are 4 "S"es to be concerned with here: Scan Speed, Sensitivity and Selectivity. Not many scanners or receivers excel in all categories. As an example, an Icom R-7000 is fairly selective and sensitive yet scans very slowly. My BCT-15X is a fast scanner but its adjacent channel rejection leaves a bit to be desired.
Some software packages (e.g., Spectrum Commander) allow setting up a primary receiver and one or more secondaries. This is what I've done. Each of the R-7000s I've deployed in the setup has had IF modifications - replacing the stock Murata "cube" filters with better quality CFJ units. Two also have SGC ADSP units installed, though an external unit such as a DSP-599 works here too.
My fast scanners - the primaries - are Optoelectronics Optocoms. A Pro-2035/2042 w/ OS-535 is an equivalent. Pro-2006 w/ OS-456 boards are another - albeit slower - option, are are PCR-100/PCR-1000s. Other supported receivers can also be tried.
Each "pod" has one Optocom, one PCR-1000 and one R-7000 connected. The R-7000 uses one of the CI-V ports on the Optocom for its interface so I can save a COM port. A custom Squelch Detect adapter allows stopping on a busy channel. The systems themselves are WinXP-Pro running in VMs on a Linux hypervisor.
I configure the Optocom as Primary and the R-7000 as Secondary. If a lot of spectrum is to be covered the PCR-1000 can be set up as Secondary 1 and the R-7000, Secondary 2. Program a Search range of, say, 118-136MHz with the appropriate channel spacing as the Step Size then let the Primary search. When it finds an active channel a handoff is performed to the Secondary (or Secondaries, in sequential order) and searching continues.
VHF Air gets one dedicated pod when scanning and UHF Mil Air gets two - one for each portion of the band. I have a total of six pods and a handful of Pro-2004/OS-456 scanners to throw at the problem if there's something major going on. Additionally, a pair of NRD-525s w/ V/UHF converters can scan known area airport frequencies or monitor the distress frequency.
Not sure about contemporary SDR software packages and what they allow. I'm planning to work with the developers of a couple open-source packages to add scan, search and multi-radio capabilities to their offerings so we'll have more options still.
