Yep, I've been using a single 60w bulb for 2 years now in my ferm chamber that holds 4 carboys. Just make sure you leave enough space between it and your pink foam (if that's what you use).
Also, 2 100w is overkill as mentioned. If you are really worried about one burning out, use 2 25w, or 2 40w bulbs. I guess it will also depend on where you ferment, I ferment in a 60* room, so I don't need to heat much, if you ferment in a 42* garage, you might need more.
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