You came up dry because not many folk waste their time on balljoint stuff. Limited range of motion and a repetitively weak suspension compared to after market link pin. That doesn't make ball joint beams worth dealing with. To answer your question......
You'll need two different size tubes and something to space out the tubes an two shock towers, either attached to the beam or to the chassis.
You'll need to be able to cut the tubes square and have a real flat table to weld on. Other than that it's pretty easy.

Everything I've been able to figure out about stock stuff is a bj is stronger than kp in stock range of motion, kp is stronger than bj if your cutting the stops off and trying to get more travel cause the bj will max out quick and kp can't max out the same. But the king pins will get sloppy real quick if you run in rough woods a lot. As for aftermarket kp, if anyone thinks a stock bj is stronger than a aftermarket kp combo Spindler they been hit with a stupid stick ALOT!!!! Lol stock kp is 5/8" and aftermarket kp is 1 1/2"

I have put two BJ front beams together a couple times and they worked well. Let me qualify that by saying I was not into crazy jumping, etc. just climbing and trail riding. I agree that aftermarket KP parts are no doubt superior, but pricey. If you just want the wider for stability, tire clearance, etc. and have two BJ beams you can accomplish this with sweat equity only. You will need two complete sets of torsion springs and cut them to fit, either put in 4 adjusters or cut and rotate the stock anchors a few degrees. A new set of ball joints should last the life of a buggy. On the other hand, if you are building this to be a top notch, second-gear climbing high HP beast and are going to be airborne up mining highwalls, then you might want to put the money in the aftermarket KP front end and basically buy everything with no fabrication needed, other than resetting the front spacing on your buggy, which would be easier than putting two front beams together. This would have more travel and adjustability. My pocketbook and nutsack are not big enough to justify the KP stuff, but I see why some of the gonzo guys (blown Subys and Eco's, 2500's, ) need them. Just depends on how you intend to use it, I put my first double BJ front end through a lot and really liked it. Also had 2" drop (raised) spindles on it, all homemade. Good Luck either way, these questions get a lot of opinions and experience brought out.

Also one other note, when cutting the torsion springs you will have to use a cutoff wheel on a grinder and try to keep them as cool as possible so you don't screw up the heat treat. These are spring steel and will eat Sawzall or other blades.