Pete, if he is building this for Service Rifle competition, we are dealing with a mininum 4 1/2 pound triger pull. I want mine closer to 5, because when I go up to the trialer at Perry to get it weighted, the last thing I want to do is have it fail and have to tear it apart and try and de-tune it.
I have mine set up with 4 1/2 on the first stage and 1 on the second. The idea is to take up the first stage and hold it while you get aimed, then break the second. When you have the first stage pulled, its then like a single stage to finish the shot. If your trying to pull through 5 pounds while aiming, its hard to stay on target. There are some very good shooters who do use a single stage in a Service rifle and do real well, but they are very few. For a match rifle (space gun) where you aren't dealing with the weight requirement, I would think a single stage would be better, but I don't shoot one.
On my Palma and long range bolt guns, I use a single stage and they are less then a pound.
So IMO it depends on what your shooting, I think it makes a lot of sense to use a two stage for Service Rife, which is why probalby over 98% of these shooters use them in competition.