Wayne-
Sounds like you are asking for two different cuts. one a parallel miter cut, the second a parallel cope [fishmouth] for round tubing. For square or round parallel miter cuts the procedure I use is straightforward. Determine your angle of miter, set your chopaw fence to that angle and make the first cut. Endo the piece for the opposite end miter cut, set it in chopsaw then level two planes: first level the length of the material horizontally, then level the first miter cut vertically. This will produce parallel miter cuts in either square or round tubing. I use the seam of the pipe on round tubing as a general visual reference. Using the center finder and a punch mark as mentioned in another post will work as well but I find it too time consuming.
To make parallel copes for round tubing [fishmouths] I have built a jig to fixture the pipe. I use a Bailiegh TN 250 for cutting copes, it is similar to the Harbor Freight Joint Jigger in design except it is designed 20 times stronger, more accurate and durable at only about the same factor in price. Very nice tool, but even if you are using a less expensive hole saw fixture tool to do your cuts, the principle is the same for the parallel coping jig.
The cutting tool is mounted to a work bench. The jig is a clampable length of 5/8" X 4" flatbar with two lengths of 3/8" sq. stock welded to each edge to form a sliding track. Into the track is a length of 3/8" X 3" flatbar that will slide in the track. To that 3" flatbar is welded a pivoting device with a threaded adjustable rod to allow for height adjustment for different diameter round tube/pipe to get to center. At the top of the adjustablly threaded, pivoting rod a short length of 1 1/2" pipe. I am using this pipe diameter as an example, the short length of pipe simply has to match the material diameter you are working on. After making the first cope, endo the piece and align the cope with the short length of pipe on the threaded pivot. When it is aligned, tighten the hole saw tooling [in this case a radial vice on the Bailiegh tool]. Make second cope, it will align the copes parallel to each other regardless of angle of cope as the short length of material on the end of the threaded rod will rotate to match angle of first cope. It is difficult to describe in sufficient detail with words, a diagram would be more usefull, but if you have a lot of this kind of work to do this might give you a starting point for some ideas. If it's only a few cuts/copes go with a level and a center finder/punch mark.