I wouldn't worry about getting a dual in case one of the siphons break. If it is set up correctly, the siphon simply won't break. The two conditions to ensure an easy night's sleep and a worry free day at the office are proper flow and wrapping gutter-guard netting around the siphon inlet, extending it all the way to the bottom of the internal box so a turbo snail or such can't crawl into the tube. You can secure the gutter guard w/ nylon ties. You may search for documented instances of a conventional HOB round tube siphon breaking its siphon and causing a spillage disaster all on its own - but I don't think you will find any, I've been looking and asking, and veteran online reefers can't come up with a single name, place or date this has ever happened to anyone. Just a reefing urban legend due to paranoia. Of course you should place something above the inner box to block out direct light from the tank lamps, and clean aglae build up from inside the tube regularly, but even a thick layer of scum won't break the siphon. If you get a dual siphon model and your return pump doesn't provide sufficient flow-thru, allowing bubbles to accumulate, then you will have a disaster, not the other way around. On a single, if you have too much flow, you can throttle back w/ a valve on the pump's outlet. If you get the double and your pump isn't up to the job, you can just pull one of the siphons out and set it aside for when you upgrade to a larger tank. Having the larger inner box from a dually will give you more leeway in terms of algae build up between the teeth strainers.
Good luck.
