If you connect scanners in parallel to one coax you will get degradation in sensitivity depending of what frequency band the other scanner are using. If they are on the same frequency band you will probably see that the signal strenght are cut in half, a 3dB loss, when the antenna signal are evenly distributed to the two scanners.
If one scanner are on 800Mhz band and the other on 400Mhz it might be very little loss compared to using only one scanner.
It only costs a T connector so it is a cheap solution. If you use passive splitter, a CATV splitter costs $5, it will interact between scanners much less but you will also have at least a 3dB degradation at all times.
Using an active tv splitter might work, they can have high internal noise and people use them with scanners where they often will give a too high signal level and overload the scanners and desense them.
Using a real multicoupler will give less problems but the cost will be something like $200.
/Ubbe