I had not thought about the bowline-on-a-bight creating two loops, one for each leg. The image in my brain was of one big loop of doubled rope creating a place to place one's big booty. Utilizing the two loops is not a bad idea; however, out of habit I would still prefer to stick with my old reliable Swiss Seat. If it ever stops raining here I'll go out and try the "two loop" method just to make sure that I'm not being unfair toward something that might end up having merit.
The Swiss seat when made from rope, as we have been talking about it here, can be quite uncomfortable and even painful. The Swiss seat, as with some other harness arrangements, can lead to "Harness Induced Pathology" or H.I.P., which is the medically correct way of saying that, yes, it will cause your legs to "go to sleep". A Swiss seat made from rope will probably be more likely to cause H.I.P. than one made from wider material. I agree with Dan completely that the wide webbing makes for a better, safer, more comfortable, and less painful Swiss seat.
The Swiss seat, as it is being discussed here for use in a minimalist approach to climbing, is something that I would never use for really long or high climbs except in an emergency situation. As stated, I would use it only for "scouting" purposes.
There are many ways to tie a Swiss seat. It seems that wherever I go I find different opinions on how one should tie the thing. Dan has detailed one way of tying the thing. The U.S. Army has their way. I have another that I use. None of us are wrong, they all work. It's another one of those things that qualify as "personal style".
When tying the type seat that I use I start with about a twelve-foot length of one inch tubular webbing. I then find the center and place it in the center of the small of my back. Since I am left-handed I will pull an extra foot of the webbing to my right. Right-handed people should pull an extra foot of webbing to the left. You end up with the webbing coming around your waist with one side being about a foot longer than the other. Right at your belly-button, tie an overhand knot in the webbing. You will now have the webbing around your waist, an overhand knot at your belly button, and two loose ends hanging downward. The loose end hanging down to your left will go under and around your left leg and come back to the front. The loose end on the right will do the same thing under and around the right leg and back to the front. I am now holding the two ends of the webbing in front of me. Since I am left handed I will then pull these ends over to my right hip and tie the seat off with a square knot over my right hip. Right handers will do the same thing to their left. I place half hitches on either side of the square knot to back up the square knot. The tag ends of the webbing get stuffed inside my waistline or into a pocket. Tying the seat off to the side keeps the knot out of the way of the main connection at the front of the seat, and allows the control hand to function without the possibility of becoming snagged in the knot. Connection to the climbing system is accomplished by placing a carabiner over the overhand knot at the front of the seat and between the two bits of webbing going downward and around the legs. The piece of webbing that comes around from behind the legs should also be within the carabiner. The seat is now ready for use.
For use as it pertains to the "one-rope" method described in this thread keep in mind that I am using the end of my climbing rope and I want to be sure and leave enough rope coming out of the harness to create my bridge and tie my Blake's hitch. Also, when used in this manner I will tie the harness off at the front rather than on the side so that when my weight is on the system I will be hanging "centered". Use of the climbing rope also implies a higher level of discomfort than one would experience using webbing.
Now. About the issue of pain and suffering: Mention the term "Swiss seat" and you will usually hear a lot of groans and war stories about just how terrible the things can be. I would like to say that most of the complaints come from people who are tying them too tightly. I tie mine snugly, but not overly tight. I won't lie to you and tell you that it is comfortable but neither will I accept the commonly accepted idea that a Swiss seat, in order to be safe, must be tied uncomfortably tight. Comfort is a relative state and I am willing to accept a small degree of discomfort in return for being able to leave a heavy harness at home.
On the issue of safety: I learned SRT ascent and descent over forty years ago and the only thing I had was a Swiss seat, tied exactly as I have described here. I have never had a problem with the thing. I will point out that when using a Swiss seat, I don't try to be "cute"; you won't catch me swinging wildly about just for the fun of it and you won't catch me doing upside-down hangs. When I use a Swiss seat, I am using it solely as a tool for ascent and descent, not for thrills. I do not recommend use of the Swiss seat as an "idiot-proof" means of harnessing oneself for a climb into the vertical environment. It should be employed thoughtfully. The possibility of Harness Induced Pathology is very real when using an arrangement of this sort. If at any time the climber begins to experience a loss of feeling in lower extremities, do whatever is necessary to take weight off the harness by standing up in a footloop until feeling returns, moving to a limb, or come back to the ground. Allow the feeling to return gradually.
Once more I will state that I do not recommend use of this technique to anyone. Those choosing to employ a Swiss seat do so entirely at their own risk. I would also suggest that those choosing to try such an arrangement do so very low and very slowly until sure that it is being done correctly.
Everybody have yourselves a happy Thanksgiving!!!