Yes, or calculate the total area and multiply that by the pressure.
If you have one 100mm and one 50mm cylinder you have an effective area of (50*50*pi/100) + (25*25*pi/100) = 78.5+19.6 = 98.1cm²

Just remember that if the cylinders aren't of the same size (diameter) they won't share the load equally (one will "push" more than the other at any given pressure).

/Henrik.