Yes, they are required to keep you sat together, but that may mean you losing your reserved seat if someone else books the other option(s). So either book both seats or book none, I don't see the merit in a halfway house since there are another 50 or more passengers also selecting seats. Particularly SFO, a very popular route. If you're travelling on business days, you are more likely to end up in two middle, middle seats, which are unpopular with single passengers but perhaps very suitable for parents.