As EOSIO protocol says, the eos can support multiple chains, how can I setup a side chain and communicate with it, I've saw dan's paper, but, what's the technique way?
You can create your own sidechain with your own private EOS blockchain,for that you must have chain running on different chainId from Mainnet and you can select the block producer from 21 mainnet block producers or you can validate transaction by yourself however, should be able to perform cross chain communication one day so in theory, the side chain can communicate with the main chain.
oh, it's easy. Perhaps you find it hard cause you don't know that issue tokens is a process to create sidechains. This's link could summarize for those steps. https://congnghebitcoin.com/threads/common-steps-for-creating-eos-account-and-issue-token.1204/