From my understanding of merged mining (I am no expert),
For merged mining to work, the "aux" chain has to accept the parents chains POW hash (that meets the "aux" chains difficulty target) as proof of work and thus generate a hash (difficulty does not matter) for its chain, with the parents chain hash in the first transaction as the proof of work rather then the hash of the block itself.
So the "aux" chain has to be patched to act as an aux chain and the parent chain has to be patched to recognize the "aux" chains difficulty and pass any hashes that meet the "aux" chains target to the "aux" chain.
So, corporation among coin developers is needed, unless one forks both chains themselves, which would be difficult to do and get enough miner support.
There is also the issue of the difficulties with the new chains, if the parent chain difficulty drops below the "aux" chain difficulty, well I guess merged mining would stop until the parent chain difficulty surpassed the "aux" chain.