You have hit on one of the most contraversial topics on FT. If we added crying babies, and seat recline I think we'd have it all covered. Am I missing anything?

I'm going to try to leave my personal opinion on the topic out of it but just answer the question. Technically, I think you're free to put your bags wherever you want (either in the overhead or under the seat). I don't think there's anything that specifically states that you are required to put one bag under the seat in front of you if you have two. However, it's also a federal regulation to follow all crew member instructions.

So, as in this case, if you would have argued, they probably could have had you removed for not following the crew member instructions.

To the OP, why not put the briefcase under the seat in front of you for taxi, takeoff and landing then just pull it out and place it against your seat so you can stretch your legs out? I'm not as tall as you but that's what I do and it seems to work pretty well.