Originally Posted by
Hannibal Lecter
You will be able to book
TXL/FRA-LHR-MAD-JFK for the same price as TXL/FRA-MAD-JFK
That way you can try both BA and IB short-haul Business and
needless to say, earn more Avios and visit more lounges

In my case, the IB outbound turns out to be even 40 EUR cheaper than any BA flight via LHR on the date I need.