dual looks best.
i reckon bells do best with a basking light, in a small/tall tank like that they will be ok but the whole tank will tend to heat instead of giving them a gradient/hot and cool areas not that that matters alot for bell frogs, its just less natural. in saying that i have used 40watt basking bulbs in exo terra clip on lamp above same tank before. small halogen bulbs possibly enough for basking.
Below is what i use for whistlers, low power consumption, not alot of heat but enough for them to bask if they wish.
(i removed glass, snapped plastic stick thing off, put on top of same tank as mentioned above in conjucntion with a fluro tube for plants).
http://www.mitre10mega.co.nz/shop/light ... te_101263/
EDIT sorry thought you mentioned 45x45x60 tank above. Which is the one i have and is not the best for bells.
Dual unit be best for that tank you have, or make something yourself? Id just put a fish tank light and exo terra basking light (or 60watt desk lamp) myself.