he will do same at his wallet, and we exchange each other's privkey generated from above, then importprivkey.
and if I input coin to there, that become multisig, both signing need and safe.
Is this true?
MultiSig addresses are created using "public" keys... not private keys. refer: https://en.bitcoin.it/wiki/Multisignature#Creating_a_Multisignature_Address_with_Bitcoin-Qt
There is also an example of a 2-of-3 MultiSig here: https://gist.github.com/gavinandresen/3966071