combinerawtransaction ["hexstring",...]

The combinerawtransaction RPC combines multiple partially signed transactions into one transaction. The combined transaction may be another partially signed transaction or a fully signed transaction.

*bitcoin-cli help combinerawtransaction

combinerawtransaction ["hexstring",...]

Combine multiple partially signed transactions into one transaction.
The combined transaction may be another partially signed transaction or a 
fully signed transaction.
Arguments:
1. "txs"         (string) A json array of hex strings of partially signed transactions
    [
      "hexstring"     (string) A transaction hash
      ,...
    ]

Result:
"hex"            (string) The hex-encoded raw transaction with signature(s)

Examples:
> bitcoin-cli combinerawtransaction ["myhex1", "myhex2", "myhex3"]