RampRat - I agree that separate tickets means no liability. The question is how to sell it as 2 tickets, but still be able to avoid the legal risk of the CAA or the courts deeming it to be a transaction involving a single ticket. After all if you market something as suitable for a particular purpose (ie journey with a connection) but actually it proves unsuitable, then lawyers start smelling claims. A reputation for dodgy products is perhaps something Ryanair has been trying to avoid in the last few years.
Yes you can set up a travel agency which is 49 % owned by each of 2 airlines and the remainder by some suitably acquiescent other unrelated party, but I'm not sure if the courts would accept this as passing the test of independence (and consequent risk of liability)