It can run two, BPB simply choose not to. For it to run 2 in an effective and efficient manner, it would require batching bays, extra staff and they would have to seriously up the pace they move about at. Seen it done in the RollerCoaster days, but with the station layout and attitude of the park these days I will consider streaking myself and racing the ride down bond street if it publicly runs 2 trains again.
It's likely to be used for rotation purposes, and if one developed a fault they have the option to change trains rather than run at a reduced capacity or close the ride.
All that said, this is one thing I would love to be proved wrong about, I would be a very happy chap to see it return to a 2 train service!