I just saw a video on this light on YouTube. Looks really nice, just expensive. I don't have one as I'm in the process of choosing equipment to set up a 29 gallon planted tank. How many you need depends on how you will install them and how far above the tank they will be. The higher the fixture, the more the light will spread (as in light up more of your tank) but it will get dimmer at the bottom of your tank. Based on the video I saw I would say a 75 needs two fixtures. For the 35 you might be able to just use one, but it really depends on the effect you are trying to achieve.
Keep in mind that what I'm saying is not based on my experience but on watching this video:
so grain of salt and all that...