Here is what I was thinking of doing

********************************************************
MESSAGE	DATA	0x50		; byte containing previous message
MUX_BIT_NO	DATA	0x51		; byte containing the position of the first bit to be changed

BIT_1		BIT	0x08		; first of 2 bit change
BIT_2		BIT	BIT_1+1	; second of 2 bit change
-----------------------------------------------------------------------------------------
MOV	A, MESSAGE			; reload the old message into ACC
MOV	A.MUX_BIT_NO, BIT_1		; change first bit
MOV	A.MUX_BIT_NO+1, BIT_2	; change second bit
MOV MESSAGE, A			; save new message
RET
----------------------------------------------------------------------------------------
END
*******************************************************

Let me know if you think that this might work, or if there is an easier way to solve the problem