Well ... a long run of coax cable attenuates the signal but doesn't otherwise disturb it. In theory at least.
A homebrew switch implies that the coax transmission line must be broken somewhere. Such a disturbance WILL set up reflections in the cable which could smear the digital signal transitions in the cable feed (=> jittery signal).
If you make the switch so that the size of the "broken part" is small relative the shortest wavelength contained in the signal, then the effect can be assumed to be modest.
But you will have capacitive coupling between signals and ground as well as capacitive coupling between the signal conductors themselves.
Video switches are (or at least ought to be) made to minimize reflections, signal loss and coupling. I remember one type of VHF switch using a slider that allowed the two signal feeds to be maybe two inches apart. Modern ones seem to be more compact.
So a VHF (TV) type switch is what you want. If you make it yourself, which I am ALL for, keep the coax shields intact all the way up to the switch terminals (and then some). Use a switch that separates the cable feeds. I think a homemade box would work. Let us know the result!
Jon
PS A google search should probably contain words like Hamradio, VHF, antenna switches, reflections, loss...