I've done three SxS's now for myself and a couple of buddies to use for cowboy action. What I found is that it's entirely doable with common hand tools. And despite not using fancy gauges to square up the muzzles the guns shot just fine to point of aim.
Bearhunter's points about losing any choke is valid though unless you're just trimming a small amount off the end to remove some damage. If you're making a coach length gun they WILL be cylinder choke from then on.
For cutting a hacksaw works just fine. As would a chop saw or metal cutting bandsaw. To hold down the amount of filing needed I'd suggest a wrap of tape that comes around and meets itself as an eyeball guide. After the cut I dress the end with a large single cut file. Alternate directions often and use the file marks as a guide to producing a flat end with no dips. Or, since good file practices are not that common, just stick it up against a belt or disc sander with a light touch to flatten the end. From there get it square by eye and with the aid of an adjustable machinists square. The barrels are tapered so you can't actually square it to anything. Instead you're looking for the ruler that extends down about 6 inches to touch the side of the barrel at the end and then look for identical gaps in the corner at the muzzle. "Looking good" according to a slightly fussy eyeball is close enough for it to shoot accurately as shown by testing the coach length barrels on patterning boards. The square is used to check for both directions.
I prefer a file for the fine work even if you start off with the rough squaring on a power sander with metal capable abrasives. It's too easy to start off with a good two inches more than the legal minimum and end up with a prohib device if you rely on the power sander. A classic case of "evening up the legs of a dining table"....

So switch to a file and work slow for the last bits. A little goes a long way. A broad flat file is the weapon of choice. And choke up on the hold and feel for running it "flat" with short strokes and more pressure on the area needing trimming. With a little practice you'll get a nice flat cut. Or work with shims and fences on the power sander to do the fine adjusting in a way that is controlled and won't quickly sand away your "safety margin" of metal.
There will be a little hourglass shaped opening in the end between the barrels and ribs. I've seen a lot of the home cut coach guns with the opening left. And at least for a steady diet of trap loads there isn't an issue. My own OCD'ism can't abide the unfinished look. So what I do is shape a little scrap of mild steel to fit into the end. The end needs to be cleaned well of any corrosion and junk so that bare metal can be seen for about 3/32 to 1/8 (2 to 3mm) into the opening. Small jeweler's files work well for this. The plug is then shaped to fit about the same 1/8 inch into the end with any gaps about the thickness of printer paper. I then bind the ends with wire and shims to hold the top and bottom ribs in place and with a propane torch solder in the plug. The excess is cut off and dressed with the file or a final run over your own shimmed rest to square off the end and flush up the plug.
Finally drill and tap for the sight bead.
It's not a hard job and I think the last one I did took about an hour from start to end.
With that description it's now up to you to figure out if it's something you can do. If you see metal files as something for rounding over stuff and if you don't know the proper techniques for filing to obtain a controlled flat surface then I'd get a smith to do the job. Otherwise it'll be a case of the dining table legs all over again.