Hello! 
Each nonce generated by the Web Payments SDK (Square’s new Payment Form) can only be used once regardless of if you are using it to create a payment or store a card on file. Unfortunately, this means that if you wanted to store the card on file for two merchants, they would need to enter it twice.
Square’s APIs does not currently support multi-party payment or transferring funds directly from your account to another Square account. If you wanted to go the route of accepting funds to your own account before distributing them to the merchants, your app would need to handle the distribution of funds outside of Square.