Originally Posted by
mmff
Where is this written?
I think the closest you'll find in writing is this in the T&Cs (my bold)
3.1.6 easyJet is a point to point carrier and does not operate a connecting flight service for onward travel using Our Flights or the flights of other carriers. If You have booked an onward Flight with easyJet, this represents a separate contract of carriage.
It's there for them to avoid liability for missed connections, but it would seem to follow (I'm not a lawyer) that outward and return flights would also be separate contracts of carriage.
As mentioned above, if you want more than this I think you'll have to contact U2 directly.