The simplest approach here is to use a conditional formatting formula in cell E5 to validate that values in both C5 and D5 have been selected. If the length of both selections is 2 a conditional formatting formula for cell E5 could be: Formula Is: =LEN(E5)<4
Alternatively a more complete formula could be: Formula Is: =OR(C5="",D5="")
The format can then be applied such as coloring the cell red or text, border highlighting as desired.