^^^ what he said.
The important tidbit is that if you have two devices which require antennas, you don't want to have the antennas (antennae?) near each other. Having two in close proximity (on the same masthead, for example) is A Bad Idea - I don't understand the physics of it, but the short version is that a transmission from one will be grabbed by the other and can fry the associated electronics.
The splitters referenced above solve this problem elegantly - not just by allowing you to have a single masthead antenna shared by both VHF and AIS, but by electronically isolating them so the AIS is protected when the VHF is transmitting, and vice versa.
I assume that VHFs with built-in AIS do the same.
If you do go with a shared antenna, it's worth looking at the ones (e.g., Vesper's) that are designed to work for both frequency bands. A normal VHF antenna will work for AIS, but being designed for VHF frequencies, it won't also give you optimal AIS performance.
Bruce