How about a paper template marking fixture, using a "laser pointer" to simulate tube #1, and a mandrel the same diameter as tube#2, with a paper wrapped around?
The laser "tube" can mimick any diameter, and be rotated by hand. The paper holding mandrel is on a pivot, to duplicate the required angle of intersect; both "tubes" centerlines intersect. Make as many pencil marks on the paper as you think necessary for the precision of the plasma cut. Also, the paper holding mandrel must be able to be rotated 180 degrees, to get the opposite side of the cut, depending on the type of intersect.
Use "teach mode", and a pointer in the spindle, to follow the paper template on the real tube, using the rotary table, and you get the real program, without math, or CAD programs. In addition, if desired, you could draw an "offset" line from the laser marked path, to account for plasma cut width. No math; no CAD programs; no file manipulation; no "Big Bucks!"
John