I've thus posted a query asking the ticketing experts over at
uk.railway to clear up any confusion (the thread is entitled "Allowed
to combine season tickets?"). Once I get a definitive answer on that
issue then we'll know whether Jonathan's two ticket hypothesis is
correct or not. And whether asdf needs to start masticating on a bowler
or not.

I rather suspect you will find that you will get three answers:

1) A TfL period travelcard *is* a season ticket for the purposes of that
rule.
2) A TfL period travelcard is *not* a season ticket for ditto.
3) Despite the question coming up repeatedly in the past, there is still no
definitive answer.

Reading the thread in uk.railway, one person asserts that 1) is the case and
this has been established many times before on the group. However you will
also find if you look in the archives that one person queried WAGN directly
and was categorically told that 2) was the case and I've also seen this
asserted several times before.

It certainly seems to me that the distinction was meant to be made between
season tickets issued by the franchised rail operators and those issued by
other operators and I would have thought TfL was meant to be bundled in with
the latter. However the distinction was made poorly and hasn't been
clarified.

G.