Atomic transfers is good concept. But I couldn't imagine combining different hashing power? How could it be? They are basing on different algo, block time, pos or pow etc. Probably underlying exchanges of different altcoins when transferring them is a bit realistic at the moment.
research merge-mining.
its been a reality for years. although mining pools wont tell you that, as they want to keep their altcoin rewards a secret.
although merge mining is not exactly the same as the concept of AT or sidechains.. merge-mining is an example that while mining bitcoin you can also mine altcoins