You can’t broadcast an incomplete transaction and have one other person modify it later, as a result of an incomplete transaction will not be legitimate and invalid transactions don’t propagate on the community.
You possibly can share an incomplete transaction out-of-band, e.g. on a message board, through a Partially signed Bitcoin transaction (PSBT) iff the inputs and outputs of the transaction creator are ample to implement their anticipated consequence through the use of a SIGHASH_SINGLE | SIGHASH_ANYONECANPAY development. The second participant would then be capable to fill within the remaining inputs and outputs to attain their supposed consequence as well as.
Nevertheless, if you need two members to each have enter on one output script, your protocol would require at the least one spherical journey, as the unique transaction creator can not 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 totally perceive what you are attempting to attain, and due to this fact could also be lacking a workaround or trick, however the above ought to cowl the overall state of affairs. Your downside jogs my memory of how inscription market locations use PSBTs to supply NFTs on the market, and in case you are not acquainted, it may be fascinating to review that strategy.
In case you typically wish to higher perceive how HTLCs work, it’s your decision to check out Elle Mouton’s glorious HTLC Deep Dive, her different weblog posts can also be fascinating within the context of your line of inquiry.
