I woudn't put any more. If you are not so sure just drive it for a few days and see for yourself how it cools down.

If it isn't working good enough with two cans it means that there is another problem somewhere else. Additional charge of freon won't help you. At certain point additional freon will reduce an A/C performance.

Why did you charge it at a first place? Is it leaking or you disassembled A/C components and put them back?