First I would say that if all your keys are held by every node, then it's not a classic blockchain use case – in general the idea of a blockchain is to divide control over things between multiple nodes.
Second if you do want to share private keys between nodes (so that each of them can transact on behalf of the same address) you can use the dumpprivkey / importprivkey mechanism.
Finally please be aware that this setup should be used in a failover rather than a load balancing pattern. If you are creating transactions from an address simultaneously in two nodes, there is a strong likelihood of a "double spend" event where both nodes are trying to spend the same previous UTXO (unspent transaction output). Unless you want to get into manually managing UTXOs using createrawtransaction, you should stick to sending transactions from one node at a time.