well let's see..
Oh lord, First off, The hijack police will be all over this one, I want to prepare you for that..
Second, I don't have any experience with this type of set up as far as that goes, I have however used one antenna and a three way 1000 W selector switch with my CB Antenna and used multiple CB's and it worked fine that way on transmit and recieve so I would assume it would work the same with scanners.. Maybe someone else will chime in here with more experience but if you wanted to use say a scantenna for 150 MHZ range, A yagi for the 800 MHZ range and a low band antenna for the low 40 MHZ range.. Put up all three antennas, hook them into the selector knob switch, Them select the proper antenna for the proper band, I don't know why it wouldn't work that way... I'd go ahead and do it.. It's just recieve only so SWR wouldn't matter and however manyu watts the switch is wouldn't matter, I'd think it should work fine but like I said, This is just my opinion since I have no experience with this type of setup..Hope this helps..