Thank you. This helps to see more of the big picture.
How confident are you that the Aera 660 and both G5 units are sharing a common power ground? You could try adding a signal ground wire between the black wire on the Aera 660 and pin 6 on each G5. The RS-232 interface won't work without a common ground between the units.
Since you are probably using unshielded wires, you might also check that your wire routing isn't running next to any devices or cables that might interfere with the signal.
Using Aviation In/NMEA & VHF Out should work, but that isn't the best choice for your installation which has no source of Aviation In or a need for VHF Out.
Are you using V3.81 software on the Aera 660 and V8.00 software on the G5 units?
Make sure to pull the plane out of the hangar and let everything get a good GPS fix when testing this.
You might also try disconnecting the JPI unit in case it might be creating interference on the common RS-232 signal line. It is generally fine to daisy chain multiple devices on a common RS-232 output like this, but some devices might not support this. I know it is fine to connect the Aera 660 to both G5 units because I have done that many timems, but don't have any experience with a JPI unit.
You will also want to make sure that Navigation Data is enabled on the Navigation Configuration page.
View attachment 28005
Steve