Indeed, yorkie is certainly correct there.
RAIL are indeed doing a 2 for 1 offer, but this offer is restricted in its use, the conditions of which I don't have to hand.
I've invited HSTfan!!! as he is the closest one I can take along with me for this trip.
I can't remember whether normal tickets are available on this train or not, but I recall reading that the part of Blaenau Ffestiniog is certainly only open to special ticket holders.
I'll see what I can find out, then I'll post again with more details once I've been to the ticket office.