Originally Posted by
Starman
I can only conclude that the web booking engine is not quite as smart as the agent's engine, and she (inadvertently) forced a through L fare for the two segment trip, even though segment 2 showed L=0 for both her and me.
This is almost certainly the case.
Most likely what's confusing things here is the fact that the flight has the same flight number for both segments, which is NOT a requirement for a married segment.
UA1851 (SJC-IAH) connecting to UA1851 (IAH-BWI) should be able to access the same married segment inventory as the one-stop UA1851 (SJC-BWI), so what happened for the agent is the correct behavior, whilst what happened on the website is not.
The other option in this case is to book the single flight, and then call and ask for it to be split into two. Might take a few calls to get an agent that knows how to do it, but it can definitely be done.