You can not broadcast an incomplete transaction and have one other person modify it later, as a result of an incomplete transaction just isn’t 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 enough to implement their anticipated consequence by utilizing a SIGHASH_SINGLE | SIGHASH_ANYONECANPAY building. The second participant would then have the ability to fill within the remaining inputs and outputs to attain their supposed consequence as well as.
Nevertheless, if you would like two contributors to each have enter on one output script, your protocol would require not less than one spherical journey, as the unique transaction creator can not log off on an incomplete output script, and subsequently 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 need to admit that I don’t totally perceive what you are attempting to attain, and subsequently could also be lacking a workaround or trick, however the above ought to cowl the overall state of affairs. Your drawback 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 check that strategy.
In the event you typically wish to higher perceive how HTLCs work, you might have considered trying to try Elle Mouton’s glorious HTLC Deep Dive, her different weblog posts may be fascinating within the context of your line of inquiry.
