I sent an e mail to the G3X folks and got an answer that may not have completely answered my question. Being so, I wanted to toss it out to find out if what Im thinking on doing is a safe idea or not. So here it is: I will be running a Plane Power 60 amp for the primary and a Plane Power 30 amp back up on the accessory pad. I will have a field switch for each one. Additionally, the back up only kicks in when the current drops below 13.2 volts. I will be attaching the primary alternator B lead to the shunt. Im wondering if I can also attach the back up B lead to the same shunt terminal. The back up would not be producing any current until activated, which means the primary has already failed. The question is would this arrangement create some inaccurate readings. Or are there issues that would be created by this set up. Reason for doing this. If Im running on the back up, I can monitor the total amp demand, allowing me to manage the load.
Here is the response from the Garmin folks.
A current shunt only knows how to measure the current that's flowing through it. If you connect two alternator B-leads to a single shunt, you will simply be measuring the sum of the total current flow. Whether that is an acceptable result for your particular application is a question only you can answer.
From: Richard Gurr [mailto:firstname.lastname@example.org]
Sent: Wednesday, December 30, 2015 9:57 AM
To: G3Xpert <email@example.com>
Subject: Re: amp shunt
I'm installing a back up alternator along with the primary. I plan to run the B lead of the primary alternator to the shunt. Is it possible to also run the back up (Plane Power gear driven) to the same shunt. The back up will be on a field switch and inactive unless the primary alternator fails. With the back up also strapped to the shunt I could monitor the output of the back up, if activated and with the primary off line (either by failure or the field switch). Will this create erroneous data from the shunt or some other issue if both alternators B leads are attached to the same shunt.