You can’t broadcast an incomplete transaction and have one other person modify it later, as a result of an incomplete transaction is just not legitimate and invalid transactions don’t propagate on the community.
You would share an incomplete transaction out-of-band, e.g. on a message board, by way of a Partially signed Bitcoin transaction (PSBT) iff the inputs and outputs of the transaction creator are adequate to implement their anticipated final result through the use of a SIGHASH_SINGLE | SIGHASH_ANYONECANPAY development. The second participant would then be capable of fill within the remaining inputs and outputs to realize their supposed final result as well as.
Nonetheless, if you’d like two contributors to each have enter on one output script, your protocol would require at the least one spherical journey, as the unique transaction creator can’t log out on an incomplete output script, and due to this fact both the second participant would have the flexibility to unilaterally decide that output or the transaction must return to the creator after the second participant provides their modification.
I have to admit that I don’t absolutely perceive what you are attempting to realize, and due to this fact could also be lacking a workaround or trick, however the above ought to cowl the overall scenario. Your downside jogs my memory of how inscription market locations use PSBTs to supply NFTs on the market, and if you’re not acquainted, it is perhaps fascinating to check that strategy.
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