I'm afraid there is absolutely no way around it. You will either need a double unit such as this one or buy yourself a 10 gang tower extention (and another single starter unit of course)
Edit: but make sure you use the correct starter unit for the correct tube.
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Thanks ive thought about getting an extension tower...
So if i do get another starter unit with the exact same 18" bulb will it double the wattage to 30 instead of 15?