Splitting one antenna into two receivers is pretty standard and easy; I do this all the time at work.
You'll need a splitter/combiner that will pass both the GPS signal (1575 MHz) and DC power to the antenna (the receiver sends a DC voltage up the center conductor to power the amp in the antenna) Both the G5 and 430 provide DC power to the antenna, and you will need to "block" one of the power sources, so the two don't fight one another ... Use a DC block for that.
I didn't see anything in the documentation for either unit that indicates that they check for "antenna faults" ... if the antenna is not drawing the expected current (over some wide range) the unit will sound an alarm. A DC block will cause such a fault if such a test is used; no DC power will be sourced to the antenna through the DC block. Any experts out there who know whether these units actively detect an antenna fault?
I'd hook it up like this:
430 <--------------------> Splitter Port 1 | |Common port <--> Antenna
G5 <---> DC block <---> Splitter Port 2 | |
A good quality splitter is about $60; the DC block about $25.
MiniCircuits has good quality components ... I'd use these for this application:
https://www.minicircuits.com/pdfs/ZAPD-2+.pdf
https://www.minicircuits.com/pdfs/BLK-222+.pdf
And for the folks who have concerns about GPS signal strength ... for aviation applications, there is PLENTY. Good quality active antennas, horizon to horizon views of the sky. It doesn't get any better than this. (the opposite: cell phones. Compromised antennas, low power electronics, lots of nearby jammers - WiFi, bluetooth, Cell, LTE, etc)