Don't assume that the compartments are either filled at the same time or with dissimilar materials.
For example, the PRR used its TM8s (single dome tank car) to ferry used crankcase oil to its reclamation plant in East Altoona from engine houses around the system and recycled oil back to the engine houses. Tired of cleaning the cars before each trip with recycled oil, they purchases 2nd hand 3 dome cars. The center compartment was used for dirty lube oil and the end compartments were used for clean oil. So, it all depended on which direction the car was moving as to which compartments were in use.
The GATC/NIPSCO car is an insulated car, so why two compartments? The following are pure speculation, but...
<snip> I find the fact that it is a two-dome that was owned by Nipsco a
bit hard to explain. I'm trying to explain to myself the reason for
any power plant to own a two-dome car. What would a power
plant be needing that justifies a two-dome car (relatively small
quantities of two different non-standard liquids)? It has me
scratching my head to explain it.