That's ridiculous. They should only share an EP if they're filmed in the same place. The commute will cost him valuable hours that could be spent on research.
Why would an Exec be spending time on research?
They are there to make high level decisions on the direction the drama is going, and also sign off storylines, handle high level casting and staffing decisions etc. They may also approve episodes before they are picture locked etc. They will also need to be across budgets.
A lot of this can be done remotely, though I suspect it would be entirely feasible to be based in London or Wales (or mid-way between the two - lots of BBC Execs seem to live in Oxfordshire...) and commute to either location.
Unless you actually live in Cardiff or Elstree - there will be travel time either way let's face it...
It's entirely possible for an Exec to look after series in multiple locations - particularly two weekly programmes. It's probably more cost effective than having an Exec per show... Doesn't strike me as ridiculous at all, particularly as the two shows are supposed to be linked.