Store each BCD digit in its own byte.

Load the LCB digits into the accumulator and decrement it. When the accumulator underflows to FF, then reload it with 9. Use the branch to decrement the next most significant digit.

Merge the digits to print them.

BEG: MOV A,COUNT1     ; Move LSD to A
    DEC  A            ; Decrement the LSD
    CJNE 0FFh, BEG2   ; Check for underflow
    MOV  COUNT1, #09h ; If underflow, reload with 09 and dec MSD
;
    MOV  A, COUNT2    ; Fetch MSD
    DEC  A            ; Decrement the MSD
    CJNE 0FFh, BEG1   ; Check for underflow
    MOV  A, #09h      ; Reload MSD with 9
BEG1: MOV COUNT2, A   ; Save the MSD
    JP   BEG3         ; Jump to end
BEG2: MOV COUNT1,A    ; Save LSD
BEG3:

Good Luck