I don't know if any such cables exist?
Theoretically, it would be a simple nullnodem configuration on the primary cables:
TxD -> RxD in both directions
RTS -> CTS in both directions
DTR -> DSR in both directions (not like a normal nullmodem cable, as it doesn't include DCD in this cable)

Then, the remaining two missing pins on the other side wire through from the second plug and port:
DTR on the secondary cable side 1 -> DCD on the primary cable side 2
RTS on the secondary cable side 1 -> RI on the primary cable side 2
This can be done in both directions to provide all pins on both sides (if needed), so full handshaking as modems or terminals can be done in both directions if needed.
Basically it's just the merged wire split onto the second port and the missing wire(RI) added on the RTS pin of the second port.

Looking at https://en.wikipedia.org/wiki/Null_modem#Full_handshaking .

So it's basically a full handshake cable, but with the DCR(connected to DTR on the other end's second serial port) and RI(connected to RTS on the other end's second serial port).

So, basically a nullmodem with the missing signals on the second port to control?
That way, things like emulating a DCE on two DTE ports is possible?