The combinepsbt RPC implements a Combiner. It can be used at any point in the workflow to merge information added to different versions of the same PSBT. In particular it is useful to combine the output of multiple Updaters or Signers.
*bitcoin-cli help combinepsbt
combinepsbt ["psbt",...] Combine multiple partially signed Bitcoin transactions into one transaction. Implements the Combiner role. Arguments: 1. "txs" (string) A json array of base64 strings of partially signed transactions [ "psbt" (string) A base64 string of a PSBT ,... ] Result: "psbt" (string) The base64-encoded partially signed transaction Examples: > bitcoin-cli combinepsbt ["mybase64_1", "mybase64_2", "mybase64_3"]