Once you add up the two indexers , your new environment will have two indexers, two search heads and one heavy forwarder. Now to enable load balancing follow the below steps.
Enable load balancing at Heavy Forwarders by editing outputs.conf
autoLBFrequency = 40
server = \ips of indexers
useACK = true
Increase queue size
maxQueueSize = 100 MB
This will enable load balancing of data at your indexers. Also enable replication between both of your indexers. Hope this answers your question
... View more