If other currencies start using Equihash will it be possible to merge mine them together?
That's not how merge mining works
My understanding is that merged mining allows a miner to mine more than one blockchain at the same time with every hash produced contributing to each of the currencies. So therefore making each chain more secure (Bitcoin / NameCoin and Litecoin / Doge for example).
I was just wondering if it were possible / likely to happen with Zcash and another currency?
Yes thats how it works but it needs to allow ZEC merge mining. Just because it uses the Equihash algo doesn't mean you can Merge mine it.
EXP and ETH you can't merge mine and they allow the same algo.
I see that's what i was wondering. Thanks!