Welcome to FT. The price you see when you book a room for 2 guests is the normal/standard price - for non-Hilton members. Nothing unusual in the rate being higher for 2 guests than for 1 guests, in fact this is the norm when breakfast is included in the rate. As stated above certain markets also have this practice.

As a Hilton menber you just book a room for 1 person and state in the comment box "2 guests - Hilton Honors 2nd guest stay free" - then the hotel will know that 2 guests are checking in. Not an issue at all.