Hi Martin,
From my experience, the force depends on the situation and can be considerable. When I was undertaking my training in Norway to become an alpine instructor we went through many aspects of crevasse rescue including classic two people climbing team. I remember the guy who was the same weight as me, slipping down into the crevasse and me face down in the wet snow fighting to stop myself being pulled down with him (while the other Norwegian instructors stood around laughing). The slope was descending slightly if it had been flat or even uphill then it would have been considerably easier. If it had been a steeper snow slope it would have been considerably more difficult. Snow cover/depth and quality of snow also make a considerable impact on the forces you can experience.
Obviously having more than 2 on the rope makes things simpler. One person can form a clump weight while the other makes the belays, however its normal for only two people to form a climbing team.
Here in Norway a 2 person team would use two ropes one clean for the rescue and another with alpine butterflies along it as the hindering rope. This method does mean you need two ropes (which isn't uncommon for a climbing team but unusual for a mountaineering or ski team). They also have the prusiks already attached to the rope, which becomes more apparent when your face down in the snow burying your axe and then trying to transfer the weight of your buddy from your hips onto the snow anchor. If you had to tie it then yourself it would be quite a faff.
From memory, I remember we were drilled through the process and by the end of the day after pulling the guy out several times with 3:1 system (just carabiners and prusiks) I was whacked. It was hard work.