I suppose you are talking about the bottom cut of FF2 and FF3 (the part of FF2 and FF3 that will be welded to FF1, with the two angles) To get two tubes with this bottom cut handed, you actually need to make only one cut in a steel box section tube.
First, you have to plan for the length of the tube (make the cut at least 360mm from the edge of the tube you'll cut, but I'll advise to plan for more, to be on the safe side, like 400mm is OK), and cut the tube with the 2 angles (14° vertically and 26° horizontally).
This way, one side of the cut will be the base part of FF2, and the other side of the cut will be the base part of FF3, handed.
After, cut the two pieces at the right length (359.5mm), and make the 14° top angle and the 26° V cut. These are very easy to see where and how once the bottom cut is done : by positioning these newly cut tubes on the jig (or vertically on FF1 on your table : 26° toward the outsides, 14° toward the back), you can easily see their position in space, and you can easily see where you will have cut and in which direction.
I hope it is clear enough, it is hard to explain with words only.
Good luck.
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