Can I use a 9mm half (double) rope for a three-man rope team for glacier travel? What is the best way for the middleman to tie in? With an eight on a bight, bowline on a bight or butterfly with two lockers?
Many climbers do use one strand of a double rope for glacier travel. This is OK because the forces generated in a crevasse fall are generally low due to a very low fall factor. Consider one of the newer, superlight single ropes such as the Mammut 8.9mm Serenity. At 52 grams/meter it is only a few grams heavier than most double ropes, and is rated to hold single-rope falls. You can use it for glacier travel, then as your primary cord for more technical leading pitches.
Professional guide Dan Mazur, who operates Himalaya, Inc., recommends using an 8mm, 30-meter, super-dry treated rope for glacier travel. He says this rope is lightweight and inexpensive. Note that it is a twin, not double, rope.
I've used the butterfly to rope-in the middle man, but find it difficult to tie compared to a figure-8 on a bight. The old argument said always to use the butterfly because it more evenly distributes the load, making it the stronger of the two knots, but with modern ropes, which don't break, knot strength doesn't matter. Mazur recommended the butterfly, but he says that a lot of beginners have a tough time tying it correctly -- for them, he recommends the figure-8.
With either knot, the middle man will have to clip to the knot using two reversed and opposed locking carabiners. A good alternate knot is to girth hitch the middle man to the rope. For an illustration on how to do this check out: http://en.petzl.com/petzl/SportConseils?Conseil=61&Activite=60.
The girth-hitch is fine for glacier travel, but you don't want to take a hard fall on it, as it could be just about impossible to undo. The end climbers should tie in using the trace-8, same as they do for a rock climb.
Everyone should tie in 30 to 40 feet apart along the rope. Depending on the complexity of the terrain, you'll want to shorten or lengthen the distance between you. Keep in mind that if you coil the excess rope around your shoulder, as is common, you'll have to tie a figure-8 on a bight on the standing portion of the rope nearest you and clip in to this to avoid the chance of being strangled by the loose coils. A good alternative is the mountaineer's coil, which is too complex to detail here.
Usually, the more rope you have between the climbers, the more slack you are likely to let develop, lengthening any potential fall. Keep the rope taut between climbers and never carry extra loops in your hand.
Pre-rig your rope for climbing out of a crevasse by setting two prussics (or Tiblocs) on your rope, along with several runners for use as foot loops. Also, pre-rig your pack with a sling so you can take it off and hang it in the event you do fall into a crevasse.
The above just touches on the rope methods you'll need to know to cross a glacier safely. To really get up to speed, take a course in glacier travel and rescue, and read the excellent books on the subjects, Glacier Travel and Crevasse Rescue
by Andy Selters, and the Illustrated Guide to Glacier Travel and Crevasse Rescue
by Mike Clelland and Andy Tyson. Gear Guy has spoken!