If you build them that the outer dimension is under 7 3/8, then two will fit side by side on a bottom board inside the rim so they are sealed flush.
That leaves a nominal inner cavity of 5 3/4 which easily accomodates 4 frames. I build them out of dirt cheap incense cedar fence boards, which are 5/8 thickness, so I have a bit of extra space.
The ability to fit the nucs side by side on a bottom board saves a piece of hardware. The bottom boards are more stable than other style nuc bottoms, and the screen boards give some early season insight into the condition of the nuc without out opening it. Splitting a 10 frame with a insert of coroplast would do the same and let you squeeze in 5 frames. Just handy to have the nucs ability to restack.
Something to keep in the back of your mind...as you think about the divided 10 frame boxes stacked 2 high...
How will you keep the side-by-side colonies apart as you lift off the top super and remove the barriers you had created (the side-by-side inner covers) and permit the bees to go between the two colonies. If the queen scurries across to the neighboring colony, you're going to be sorry.
I think that if you have the time, skills, etc., creating the nucs to fit side by side on a 10 frame bottom board (with a divider and separate entrances) will be better for individual colony inspection. You can keep a piece of burlap on top of the colony you aren't inspecting to keep those bees from walking about where they're not supposed to go.
I'm building double divided 10 frame nucs. The bottoms just have a 3/4" divider with an extra 3/8" piece on the tops to fill in the gap at the top. The tops are deep boxes at 8 1/8" wide. They sit on top of the bottoms with each one having it's own plywood top. I may drill a hole so I can feed them directly.
The next batch I'll dado the center of the bottom box so I don't have to add that little piece. These were retrofitted 10 frame deep boxes.
Here's how you calculate a box size. Since nucs are sometimes made of 1/2" plywood (see D. Coates references on Beesource) you want to calculate the inside space and then allow for material thickness after that. A standard frame is 1.375". Take the number of frames (in this case 4) and multiply * 1.375" and you get 5.5". Now just to have minimum beespace on the outside edges and the frame only provides half of that so you have to add 3/8" so that is 5.875" bare minimum. A typical ten frame box has an excess of 1" more than the width of the frames, so that would be 5.5" + 1" = 6.5". I would consider that the maximum useful space. So somewhere between 5.875" (5 7/8") and 6.5" (6 1/2") inside measurement depending on how tight you want things. It's nice to have a little extra for when you have an uneven or fat comb you want to put in there...
Do you have specific reasons for a 4 frame? If you are using drawn comb, they fill 5 frames up fast!
I wouldn't suspect you want to put two side by side, on top of a super of bees below in your winter climate. If you plan to do this need to factor in width of two NUCs.
I built some 5 frame out of 7/16 plywood(oriented strand board). Inside dimension is 7 5/16. If you want one less frame deduct 1 3/8. 5 15/16.
This gives ample room on the side frames to lift them out and accommodate thicker comb. Don't build things real tight as it makes getting the first frame out and last in difficult. Free space is 7/16 total so when frames are centered and tight there is 7/32 on each side.
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