How close is TOO close

I just want to make sure that I'm not going to burn out my scanner by having it next to (within 4 feet) of my CB (actualy CB and scanner are about 2 feet apart bu the antennas are with 4 feet of each other). I dont scan the CB range when I key up, but both CB and scanner run all the time. I know sometimes when I key up the scanner stops in the 400something band range (a public works channel), but I half expected it to do that. So should I invest in an RF shield for the scanner or is there anything I can really do other than not making it a habit of keying up with the scanner running.