Multiple MPPT60/80 controllers have low output.
Multiple MPPT60/80 charge controllers have lower than expected output
In installations that use multiple XW MPPT charge controllers, they will not necessarily split the power evenly among the controllers.
If one charge controller can fufill the needs of the battery at a specific time, then the other charge controllers will regulate their power down.
NOTE: If you are not selling to the utility and the loads are minimal, assuming the batteries are fully charged, there is nowhere else the current can flow, which is why the charge controllers would regulate their output current.
Make sure the charge controllers have the autotrack enabled and the charge cycle should be 3 stage.