Bonsai's contracts do support multiple signatures!
At this time, you can sign once for yourself as the contractor, and you have the option to request an additional co-signer for the "client" on the contract.
You can also cc additional users on the contract, but note that they will only see a 'read only' version of the contract. Learn how to do that here: https://intercom.help/bonsai/bonsai-contracts/how-can-i-cc-or-copy-other-additional-clients-on-my-contracts