For 9" width, you are better off with a double tube -- plants on the outer edges won't get as much light and will try to lean in -- especially after they get bigger. But if you get good sunlight from the window as well, you can compensate by rotating. (Moving the pots around) -- but this can get tedious.
In other words, it's possible to start off with a single tube and add on later, but if you can afford to get a double tube fixture now, then you might as well because you'll want it later.
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