As was also suggested by @Archmage, the problem lies with your operation scheme. Having had a similar problem recently with two water heaters on the same loop, the problem should be solved by adding a PlantEquipmentOperation:ComponentSetpoint object for the loop rather than the PlantEquipmentOperation:HeatingLoad you currently have.

I'm not at all clear on why this is the case. That is, why a PlantEquipmentOperation:HeatingLoad scheme controlling multiple plant items in a PlantEquipmentList doesn't work. There doesn't seem to be anything explaining this in the documentation and I've seem odd behaviour at times so I'd be interested in a more comprehensive answer that gives the reasoning/logic behind it.

For reference, here's the section in the docs about plant and condenser control, though to me it's not clear how to work out the issue from reading it.