Load balancing between harvesters is not supported.
In NFA 10.0.x and later, every time a router switches between harvesters it is going to create a new instance of that device in NFA Enabled Interfaces screen.
It will mark the old one as OLD or Retired in the db and then you would have to merge the interfaces to keep historical data.
This process cannot be automated, so if you have devices switching harvesters frequently you would have to do lots of merging to keep the historical data.
Many customers use a third party tools to handle the distribution of flows to harvesters, however routers should generally stick to the same Harvester unless you are planning a one time move of the router.
Also note that after a router moves to a new harvester and if you merge the data and delete the old device, you will also need to perform a full resync of the NFA Data Source in CAPC to ensure the device is fully removed from CAPC.