More than likely you are going to need another needle valve, or at least some other way to control the pressure individually. It's possible that you may be able to meter them individually after the needle valve with some decent aquarium gang valves, but I can't say for sure if the ones you get at the LFS are able to handle 10-15 psi which is what most of us are running at. Without individual control you will be getting different flow rates in each tank.
Thinking more on this, I don't think it necessarily means that there's 10-15 psi after the needle valve. I also think it will depend on your diffusion method in each tank. If you are using something that requires the high pressure like the ceramic/glass diffusers I think you will definitely need a second needle valve. If you are using a low pressure diffusion, like the red sea 500 or something, I think you could use the gang valve idea after the needle valve. Keep us posted on what you find, as I'm interested in the outcome myself.