Two antennas to one scanner is generally a bad idea, which is why most folks find one compromise antenna that works best for them. Generally, if you need multiple antennas for reasons such as covering different bands, you can either use an antenna switch or a device to properly combine the antennas commonly known as a diplexer and/or triplexer (depending on your needs).
If you are thinking that if one antenna of type X works well, then combining 2 of them is bound to work better, the answer is usually no.