Welcome to the forum, Tony.
If the two signs are back-to back, you simply need to take the dimensions of the larger one (or of the smallest rectangle that encloses them both if they are different shapes). If they are one under another, the easiest (but somewhat conservative) approach is again to design for the enclosing rectangle.
If you have time (or your software will let you) it is better to design individually multiple signs that are not on a backing board, so that you don’t over state the total sign area subject to wind forces. You should sum the bending moments from the two separate calculations before comparing them with the capacity of the required section. You can also use that combined bending moment for foundation design.
Deflection calculations are complicated for two separate sign plates, so it’s sensible to use the ‘bounding rectangle’ approach when doing them manually.