I'm a newbie to aviation, but I've spent lots of time in my day job chasing down interference between different RF signals, and I personally would avoid bundling the com transmit antenna with the gps receive antenna if at all possible. By bundling those, you would be bundling a cable with a HUGE transmit signal right next to an important antenna that's trying to receive a TINY signal.
It will probably work if the shielding is good and harmonics are low, but if it doesn't, it will most likely only be a problem when you transmit on certain frequencies whose harmonics land near the gps signal. Given that you won't know the problem exists until you try to use those frequencies, I'd personally try to put some distance between the two antennas.