For glacier travel, many experts feel that four is an optimum number on a rope team, with three slightly more risky. If you choose to travel as a two person team, each climber needs to be highly skilled in crevasse avoidance, crevasse rescue, and have all the necessary gear.

A two person team is harder, because stopping the fall and then trying to build an anchor with the weight of your fallen partner on your harness is a significant challenge.

Studies by ENSA (École Nationale de Ski et d'Alpinisme) or French National Mountain Guide School, determined determined that a few bulky brake knots in the rope between a two person team can significantly help with crevasse rescue, assuming fairly typical snow conditions on the surface - not completely bare, and not too loose and fluffy. The knots typically shorten the length of a fall, and make it easier for the person on top to hold the victim.

(On the flipside, the knots can complicate prusiking up the rope and rigging a mechanical advantage system, but the benefits of a shorter and easier to catch fall generally outweigh these shortcomings.)

The short version from ENSA:

“Our tests validated the effectiveness of this technique, and we strongly recommend climbers use it.”

While a standard figure 8 on a bight loop or butterfly knot is effective, ENSA suggests using a “brake knot”. It creates a larger diameter, more spherical-shaped knot that offers more friction against the snow.

Here’s the method they recommend to tie the knot. Fortunately, it's a simple modification of the figure 8 on a bight, so it should be easy to learn and remember for pretty much anyone.

Tip - Don't make the loops too large, because this is just wasting rope.

Each brake knot takes about 1 meter of rope, so take this into account when setting your rope spacing. Tie the knots first, and then measure your 7-8 or so arm spans between climbers.

Start with a standard figure 8 on a bight, with a loop of about 1 foot.

Next, tuck the loop around the knot . . .

and finally, pass the loop back through the knot, then snug down each strand to dress it. The final loop created should be just a few inches tall.

The knot tying instructions starts at 6:20 In the video below, hopefully this link should take you right there.

And here's a nice video from Ortovox featuring some pro German guides who have much the same conclusion. It works!