I have special top bars for them, but have only used them a few times. I usually use foundationless frames (or whatever I can get my hands on).

I originally built all my long boxes to fit the equivalent of 4 deeps, now I just do a 32 inch box (3 deeps). You can drill a hole in both ends and get a divider and run two hives in one if need be, but I have never done it. Invariably, the extra space at the end not taken up by bees becomes a feeding or storage area - usually a feeding area. They usually have left over winter sugar in them for dearth periods.