since only code snippets posted and
no hardware description pic/usb descriptor and
no explanation why eusart not employed and
no description of failure point or why "working" code not suitable

for a meaningful forum response more detail would not go astray.

i'm only a novice at usb stuff at best but there is just not enough data to go on for me.
in general i would not expect two such asynchronous time critical services to ever be reliable on a single core device
without some hardware support.


some thoughts
SearchForClockSignal:
@ INT_DISABLE USB_INT
PIR2.5 = 0 ? why here
FOR J = 0 TO 5
SERIN2 PORTB.7, 32, 5, CONTINUESERIN2, [STR RFID_IN\7] ? 5ms ? usb can be polled at intervals down to 1ms device dependent
// what is your actual poll frequency ? use wireshark to get a picture of whats happening
CONTINUESERIN2:
USBSERVICE
IF RFID_IN[0] = 252 THEN EXIT 'THE DATA PACKAGE WAS RECEIVED
NEXT J
// no attempt here to clear irq flag before re engaging isr could be an issue
@ INT_ENABLE USB_INT

'.... SOME CODE HERE TO PROCESS THE DATA

CLOCKSEARCHFAILED:
RETURN