just a word to the wise, there are quite a few people nowadays about with 60m ropes so if you see someone abbing somewhere, do not assume it`s doable with your 50m ropes.
Yes I know about knots in the end and making your own judgements.
The opposite happened to us in the Dolomites last summer. Guide in front on a single rope, we on 60 metre twins. Guide runs two pitches together that our topo says would be more than 60 metres. "Oh", says we, "his single rope must be a 70". Uncomfortable semi-hanging belay between two pitches that, as it turns out, we could easily have done in one with our 60s.