Here is a stonewallx2 Collaborative txn using two wallets each party shares txn fee over soroban no other wallet can do this.
In a stonewall txn the dummy output goes back to the original wallet owner and the payment goes to the merchant. In a stonewallx2 this video, the tx initiator makes a payment with an output to the merchant, and the collaborator gets an output back for the same amount.
Stonewall breaks the common input ownership heuristic because it has coinjoin identical output sizes.
An observer of a stonewall tx has to decide if it is a stonewall or stonewallx2. But there is no way to tell by looking at the transaction.
https://blossom.primal.net/cfc0b331b86b52e123977894e7f4071b225459d53716585baf2e23996e739ed2.mp4