Well, my proposal wasn't to mix 2 transactions, but maybe 10, or even 50. And then once you add the change addresses (at least 1 per wallet), it is no longer so easy to figure out what was used for what, and what belongs to what.
The only problem is that the more participants in the group, the easier it is for one party to disrupt the operation OR spoil the anonymity by recording/publishing the input-to-output connections. Everyone has to participate, and no matter how you slice it, either everyone in the group will know the mapping of addresses for everyone in the group, OR anyone in the group can easily disrupt the process for everyone by failing to play by the rules (e.g. refusing to sign, or spending their original funds while the signatures are being coordinated, thus voiding the whole transaction).
The right group size is small and casual. The average Bitcoin node tries to connect to 8 other nodes, and if "some" of those nodes cooperate, you'll have a group size of 2 to 4 most of the time.