Parallel Battery Issues

[72k20Farmtruck]

Hey everyone,

I have a 1982 K20 that used to be a 6.2 diesel truck but now has a 99 vortec 454 in it. I’ve noticed lately when starting my truck, it seems to be suffering from low voltage. I do have dual batteries wired in parallel. Both were purchased ~1 year ago, same exact battery. I put my meter on the passenger side battery while the truck is running and get only ~12.1-12.3v, but the driver side shows a proper voltage of 14.6v. So i checked my alternator, voltage at the post of the alternator matches the driver side battery @ 14.6v. I also checked the current on my charge wire, it reads ~1.8-2.2A. Just for reference, my alternator charge wire is a 4awg and is soldered into the battery circuit at the passenger side battery. Could my alternator be going out? Or do we have a battery with a bad cell?
Thanks for your time.