Two 200w heaters should be plenty for a 46g...you'd probably be fine with a couple of 150w heaters. 50w is fine for the 10g. You set both heaters for the temp you want and let them cycle. If one of them ever sticks, you'll have more time to catch it before the tank overheats. If one fails to come on, the other will keep the tank from getting too cold. I'm going to give some very opinionated advice about buying heaters. If you want a basic heater, go with an Ebo Jager. If you want a really good heater, go with a Won Pro Heat II. Stay away from everything else. The Ebo is a submersible glass tube heater, inexpensive, and pretty reliable. The Won is a titanium tube heater with a separate temp probe and an IC based controller. It has a digital temp readout on the control unit. You might want to go with one of each...a 150w Pro Heat II and a 200w Ebo.
JMHO.
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