Do not quite get what you mean by 'both'. Imagine you want the radio to send the AIS signal on to the Garmin
and displayed there? Do you mean once the signal is sent out from the SH it cannot be displayed on the SH anymore? (not likely)
The Garmin (my older unit) use the 'data in' wire and the ground (negative power wire) for NMEA
The SH has pdf files for various connections, including a good set of Garmin models on their website, here:
Welcome to StandardHorizon.com
If your Garmin is not on the list pick up the nearest match and then make sure it is the same between your pick and the one on SH list. Then connect. If unsure about the Garmin use their site and read their manuals
Mind the AIS signal will likely by high baud one so set up the Garmin for adequate setting (NMEA IN and then the speed in bauds as named in SH manual).
Let me know how it went!