Double RC acceptance parameter setting is invalid

In order to ensure the stability of the equipment, two RC receivers are used on the aircraft.

Enable the 10th Enable multiple receiver support of RC_OPTIONS.

The hardware is Mateksys H743-Wing, and BETAFPV ELRS915.
But it seems that only the data of SERIAL6 is received, and there is no data of SERIAL4. Even if SERIAL6 loses signal, it will not switch to SERIAL4.
(The signals from both RC receivers come from the same remote control)
Is there anyone who has achieved dual RC reception?

Firmware version is 4.2.3