You could keep 2 public variables, say A1Ref and B1Ref, and on the
worksheet change event have something like this

if range("A1").value < A1Ref AND range("B1").value < B1Ref then
range("D1").value = range("A1").value + range("B1").value
A1Ref = range("A1").value
B1Ref = Range("B1").value
End If

Although the problem with this is if A1 changes twice before B1 changes
or vice versa, would this happen?