I understand, but if we have 3 people, I put in 5BTC, someone else puts in 2BTC, and the third person puts in 3.5BTC, the coins all get broken up into random amounts and go through two stage mix, in the end, we still have 5BTC, 2BTC, and 3.5BTC, just in new addresses. Isn't that fairly easy to track? I know I'm missing something... What is it?
You don't re-assemble them into the original amounts. Why would you want to do that?
That's what Blockchain.info does. You send from a single address, you give it an amount and a destination address, and it goes through 2+ CoinJoin steps, split up into random amounts from your source address, then after all the steps gets recombined into the amount you sent (minus transaction fee) into your destination address. Basically, it's as if your amount is exploded into random chunks, and then recombined into the same amount. Unless multiple steps involve multiple different coinjoiners, and much of what you get isn't tied to your own bitcoin at all, I'm not sure it's secure.