The two AIS frequencies are 161.975 (channel 87B) and 162.025 (channel 88B).
Both are within close proximity to the rest of the +/- 4 dozen marine VHF channels (161.975 - 162.025). So the standing wave ratio (SWR) is predictably LOW across this whole spectrum for all marine VHF antennas... and thus a "different" antenna is not necessary for the AIS channels if you choose to use a splitter.
A "resonant" antenna is best for transmit and receive. But it is only "critical" for transmit. Any commercially available Maine VHF antenna *should* be resonant across this whole frequency range of all the Marine VHF channels.
I do not know of any Marine VHF radios that have two antenna ports. But there are several AIS transponders that have dual antenna ports so the AIS transponder can "share" the same antenna with a VHF radio.
e.g. We have a Raymarine VHF 73 radio that shares the same Shakespeare VHF Marine antenna as the EmTrak B924 AIS transponder... which has an "antenna splitter" built-in. So the antenna's coax screws into the EmTrack 924, then there is a short coax cable that goes from the EmTrack B924 to the VHF radio.
This works very well. The Raymarine VHF 73 has AIS receive only. But it did not take us long on our Great Loop trip to realized we wanted to add AIS transmit. Thus the EmTrack B924 addition.
Some more details here:
https://shellerina.com/2023/09/19/ais-ing-again/
I am a big believer in AIS. Good technology. Good investment.
Hope this info helps.
Ray Sirois
N1RY (Extra Class Amateur Radio Operator / ARRL Registered Instructor)