double windows (again)

Hi everyone. I know I have posted this before, but I need (it has come to this point) to replace the double window in my shed. Do they make a double window (already mulled/joined together) at the factory and if so is it something I would have to special order from Lowe's or Home Depot? I looked at my shed window again from the inside, and the previous owner didn't finish off the inside of his shed with window casing, so I can see the rough opening and all. I looked at the very bottom of the sill, where the two window jambs that are nearest together rest in the sill. Being able to see it, there are premade notches that are cut in the sill that the jambs fit perfectly into and rest in. I took a pencil and poked between the jambs in what would be the middle of the windows, and it is hollow, so no 2 x something between the windows. So are these probably made this way at the factory?
Now after that long story, it would seem to me that it would be cheaper to by to single windows and mull them myself. Can I do this in this opening, or would I have to by them premulled again?
Thanks for any help.
P.S. If anyone wants more info, or wants a picture of the window, let me know.
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