MPC system should accept multiple payloads for signing. Obviously we can accept a longer TX time, and pay more, but it seems like most teams working with Bitcoin UTXOs are scratching their heads trying to come up with solutions for how to spend multiple UTXOs.

Each UTXO to spend requires it's own signature. So with the current configuration of MPC we can really only spend from one UTXO.

Possible solutions outside of the MPC accepting multiple payloads would be:

• chain promise calls from a contract CONS: derived accounts are based on contract, have to make a self call to contract before returning the parallel calls made to MPC back to client
• make multiple calls from client CONS: wallet / user needs to sign multiple TXs

Seeing as how everyone is going to want to do this sooner than later, for Bitcoin UTXOs and other applications, I think it should be a priority.

(requested by @mattlockyer)