I kind of suck at boolean algebra, but:
https://github.com/bitcoin/bitcoin/blob/37612099ec7314b15a07d8bac55161ed4e8e7491/src/rpc/rawtransaction.cpp#L1672


Unless I'm mistaken (which is highly likely) the whole thing can't work  (except the edge case where `permitsigdata=true` and you have no sig data) 
"1. \"txs\"         (string) A json array of hex strings of partially signed transactions\n"
            "    [\n"
            "      \"hexstring\"     (string) A transaction hash\n"
            "      ,...\n"
            "    ]\n"
            "\nResult:\n"
            "\"hex\"            (string) The hex-encoded raw transaction with signature(s)\n"
            "\nExamples:\n"
            + HelpExampleCli("combinerawtransaction", "[\"myhex1\", \"myhex2\", \"myhex3\"]")
        );
    UniValue txs = request.params[0].get_array();
    std::vector<CMutableTransaction> txVariants(txs.size());
    for (unsigned int idx = 0; idx < txs.size(); idx++) {
        if (!DecodeHexTx(txVariants[idx], txs[idx].get_str(), true)) {
            throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed for tx %d", idx));
        }
    }
    if (txVariants.empty()) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Missing transactions");
    }
    
    
    CMutableTransaction mergedTx(txVariants[0]);
    
    CCoinsView viewDummy;
    CCoinsViewCache view(&viewDummy);
    {
        LOCK(cs_main);
        LOCK(mempool.cs);
        CCoinsViewCache &viewChain = *pcoinsTip;