Finding only two seats together shouldn't be too difficult even with B17 and B18. But you'll probably have to go all the way to the back of the plane to find them. Just don't panic if you board and it seems like the plane is full; the front of the plane always looks full when you board in the B group, but once you get past the crowds, you'll see a lot more empty seats in the back.
If you wind up not finding two seat together, you can also look for two isle seats directly across from each other or one in front of the other. And you always have the option of asking someone to move, though many people will refuse to move because they got up early to check in at the 24-hour mark and get an A boarding pass, etc.
Even when I get an A boarding pass, these days I usually go straight to the back of the plane anyway, because it gives me the best chance of having an empty seat next to me if the flight is not 100% full. Those rear seats are always the last to fill up, even on a full flight, and I often have two empty seats next to me right up till the C boarding group get on the plane, so I don't think you'll have too much trouble finding two seats together.