My experience with bookies is that there are not straight up bets unless that is where they set the odds. If the game has a line on it I have never heard of anyone ebing able to place a straight bet. I am not big in the scene, so I could be wrong.
This is basically right. You will get a straight-up bet vs. the spread, but you won't get even-money on Pittsburgh to win, as you said below. If you win the Steelers bet you will win less than what you bet.
So the three possibilities, ignoring the juice, are:
1. Seattle wins. You break even (win your even-money bet on the Seahawks, lose your uneven bet on the Steelers).
2. Pittsburgh wins and covers. You lose a little money (lose your even-money bet on the Seahawks, win your uneven bet on the Steelers.)
3. Pittsburgh wins but Seattle covers -- "the window". You win a lot (win both even-money on the Seahawks and uneven bet on the Steelers).
This bet is basically a "lottery ticket" style bet. You are almost certain to either break even or lose money. But if you hit the window, you will win a lot. But it's a pretty small chance that you would hit it.