Greg, did think about that solution but had the concern that the shunt rating needed to be sized for the combined output of both alternator if both field switches got turned on together. The solution to that would be to change the field switches to a single two pole three position with center off. This has the disadvantage of only being able to have one alternator active. I have ANL fuses in the output lines for both alternators and then through the shunts to the battery common point so I could take a current measurement across the alternator side of both shunts in series. This would work if the scaling for both shunts is the same and only for the case where one alternator is active. For the case of both alternators active the reading would be undefined depending on the relative voltage output of each alternator. I have dual P Mag ignition so have some independence to keep the fan turning but dual HDX all glass displays with no back up steam gages so having knowledge of what is providing (and what is capable of providing) electrical power is important. Seems there is no obvious easy answer.
KT
The shunt doesn't need to handle the combined output of both alternators - it will only see the output going into the battery (charging the battery) which will be limited by the resistance across the battery and the charging voltage. For a lead acid battery, the most it will usually take is in the 20 amp range, while a lithium battery might take 35-40 amps initially after start and tapering off quickly.
I run both alternators occasionally as well - with separate field switches - the alternators will not output more current than is needed to maintain the voltage regulator setpoints, and as that voltage rises the regulator drops the alternator field current to bring it back down. So if you have for example 15 amps of active current needed to keep everything running, and the battery wants 10 for charging to bring it up to full charge, then you need 25 amps from the alternator(s). 24 amps would result in a slightly low bus voltage, and the regulator ramps up the field current, while 26 would be slightly high and the regulator ramps it down. It doesn't matter if you have one alternator or two online, they will simply share the load and respond to the bus voltage.
In practice, what I do is keep one alternator on all the time, occasionally switching from one to the other to keep them both exercised. My bus voltage rides at a normal 14.4, and my amps rides at a normal zero after the lithium battery is recharged after engine start (a couple minutes). I have alarms set on my EFIS both for low voltage (at 13.5 if memory serves) and high current (anything over 15 amps, I need 19 amps to keep everything running) flowing OUT of the battery - both of those would alert if my alternator took a vacation, at which time I would simply shut off the field coil on the active alternator and bring the other one online. My standard runup routine includes checking both alternators for output, and checking the alarms for function by taking both alternators offline.
Some folks will tell you that switching alternators on/off with the engine running is a sure way to kill the alternator - and that was true decades ago - but the modern alternators ramp the field coil amperage up slowly enough that you simply can't/don't shock them to that degree. I've got 700 hours of flying on these original pair of alternators (Plane Power primary and B&C secondary) with no problems at all, and I switch them on/off every flight with the engine running.