In a chain split scenario (not soft fork), a miner has to confirm transactions from both chains? In a scenario where a chain split occurs (two miners find a hash), would miners have to work with both chains until one of then become larger (more PoW)? i.e. Would they have to validate transactions from both chains as 1 confirmation until then?
No they don't. Miners only work on one chain. 

In a chain split scenario miners are divided: some work on a chain and the others work on an other chain (both chains are correct, but they have a different last block). The miner who find the next block makes its chain the correct one because it's now the longest.