The word "two" appears a LOT in the blue book.
Anyhow, here's what it has to say:
A passenger who requires two seats due to a
disability, including obesity caused by a disability,
a leg in a cast, a hip operation, may book two
seats and pay only one fare.
d) A passenger who requires two seats due to size
alone, not caused by a disability, may book two
seats but must pay the lowest applicable fare for
both seats.
e) A passenger cannot book two seats simply
because they do not want anyone sitting next to
them. If the train is full, it is very hard to explain
to standing passengers why that empty seat cannot
be used.
Short version: Need two seats because of disability? One fare, but you get two seats. Need two seats because of your weight? You can have two seats, but you'll have to pay for them both. Skinny and antisocial? Tough. You'll sit in one seat and you'll like it.