If they don't have the same state of charge voltage curve, you would need to restrict your use to the least extreme limit for charge and discharge.
Say that one has a full charge cell voltage of 4.2V and the other is 4.1V, you would not want to charge the first past 4.1V or you'll immediately be overvolting the second when they are put on the same bus.
It's not anything insurmountable, but you basically end up needing to build an entire battery system that also has to coordinate with what's already there.
Document what you do, chances are there isn't much help others can offer.
2012 Candy Blue FFE
33.5kWh battery pack
CHAdeMO DC Fast Charging