Another question is how to ensure they are on the same block.
Current design require two tx.
Make it to single tx.
Why two txs cannot reside in one block?
A ---- mixer ---- B
tx1 tx2
There is network delay.
Actually A is broadcasting to network his tx, signed by his private key.
A commuinate mixer to know which address should be used, mixer issue a address for A, and inform mixer real payee.
It should be included in a block.
There is possible way, if mixer relay the tx solely.
Hold the tx1 and process tx2, broadcast tx1 and tx2 simultaneously.
But who generate block ?
There is network delay.
tx1 and tx2 can be splitted to two blocks sometimes. no 100%.
By any means decribed in
https://bitcointalksearch.org/topic/m.7289855