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We have been taught that when a rope ends and we need to attach another one on a rebelay we pass the end of the second rope through the loop of the figure eight from the first rope and through the carabiner and make a sewn figure eight. This way the ropes are linked together and connected throught the carabiner. My question is: is this safe? we have two ropes rubbing together if the carabiner breaks and the possibility of the carabiner breaking is too far fetched so why do we do this? (It is a french system called 'the shunt'.)

what you've described, as far as i can tell, is correct. each "end" of rope should have a figure eight loop, those loops should be interlocking, and your rigging hardwear (biner, mallion, whatev) should go through both loops.

if the anchor fails (carabiner breakage is as you mention probably not likely, but if it's a bolt, the bolt could pull out, etc) you WILL get some rope-rope rub, but it will not likely move much due to the load on the rope.

it beats the alternative of not having the ropes joined...

Here is a pretty cool website that might help.

http://www.caves.org/section/vertical/nh/46/srt.html