The math is certainly in your favor. Assume you are on a 143 passenger 737-700. On an originating flight (one that was empty when boarding started), there are 24 rows with two window and two aisle seats (except the exit row with two seats has a hybrid window seat). Although many flights have pre-boarders that take a few seats before general boarding begins, there will still be a significant number empties in the 96 seats located on the window or aisle. Since most people traveling alone do not voluntarily choose a middle seat (the front row bulk- head middle seat being an occassional exception), one can consider that middle seats are generally available until the last 48 people on a completely full flight board the aircraft (theoretically), so if you find an aisle or window then you will likely find a center seat next to that aisle or window seat, allowing a couple to sit together.
The person with the B-1 boarding pass will likely have to board after a few (maybe a dozen or so) A-List, A-List Preferred, and families with small children, (and sometimes military personnel) which means that that person will be boarding in the 70th to 80th position, so mathmatically there should be 16 to 26 windows and aisle seats available, even if no one chose a middle seat up to that point. So it stands to math that the B-16 boarding pass should be able to find a window or an ailse seat.
There are some exceptions to this math approach. That math does not work as well for a connecting flight, because it is difficult to know how many window and aisle seats are being occupied by through passengers. Also, in the morning and afternoon/early evening a lot of A-List passengers that travel frequently may be in the queue headed home from business, and that too will mess with the math. Of course if you are flying on a 737-800 yopur probabilities are greatly improved because that equipment seats 176 passengers. As they say, "Do the math!"
Good luck and happy travels.
Traveler Tip: It is often a good idea to ask the Gate Attendant or one of the counter attendants how many are scheduled to be on your flight. At least it allows you to know what to expect.