Ethereum uses the algorithm Hashimoto to mine for ether, other altcoins such as Expanse also use the same algo. Like bitcoin and namecoin, can ethereum and expanse be merge mined? (and how can it be implemented?)
You can mine ether and decred but it is not merge mining but dual mining. It is not the same performance as solo mining but it seems interesting. You could find many information on the subject: https://bitcointalk.org/index.php?topic=1433925.0
Merge mining needs a special design of the second altcoin to support it. The Dual Mining is a special trick to reuse computes of ether with Decred.