C2=IF(A2+B2=0,"0",C1-A2+B2)

the above formula is which im using presently. then i came out with the one
below to eliminate the error which would occur by entering data on both
cells, A2 and B2 at the same time.

C2=IF(A2=0,C1+B2,IF(B2=0,C1-A2,IF(A2 AND B2=0,"0","ERROR")))

what i need to do is if A2 and B2 has values at the same time to show an
error message.appriciate if someone could correct the above formula.