In the very off chance that its not the same HD wallet, I just don't understand why "different users" would entrust each other with their private keys unless they were very close, or more likely, the same person.
The chance that they Gave each other the private key at the same time is near zero. The chance that one gave the private key to the other to create this transaction is near zero as it makes zero sense to do so (each could have made their own sending the same amount to the same address).
The chance that they managed to construct this transaction without sharing private keys, i.e. constructing partially signed transactions and exchanging them and making them work is zero. They do not have 1/10th of the brainpower nor knowledge to do this.