It all depends on your hardware
If your multiplexing device is the iAIS, then check if it is NMEA 183 or NMEA 2000
(N2K) input. If it is 183 then you can only multiplex one talker per each physical line (read this as one set of data, e.g. GPS
data). A N2K input will allow multiple talkers, by its own design.
Then access the data at the point that is convenient (mostly: NMEA OUT of any device) wire it into your iAIS and then you get multiplexed data (AIS data and e.g. GPS
data) on this device's (iAIS) NMEA OUT.
There is a zillion of ways to approach this cat and it is all hardware
As a side dish I will say that if you do like playing with interfaces / NMEAs and data streams then given the way the gadgets seem to be headed, I would suggest wiring
all your data into a wireless NMEA/N2K multiplexer. I say this because I see more and more devices getting rid of cables
and I believe this is where the future is.