Polyurethane on hardwood floor dilemma
You have a few options:


Don't put any more Brand B on.  Go get more Brand A and finish the rest of the floor.  After it has all cured, place a 5' x 7' area rug over the Brand B area.
Continue with Brand B over the rest of the floor.  Yes, it will look different than you originally expected, but at least it will seem like it is supposed to be that way.  Who knows?  In time you may prefer the accidental finish.
Rent a floor sander and either strip off the Brand B area, or the whole floor.  With the former, you could get lucky and be able to re-polyurethane the one section to match.  If so, stop.  But odds are you'll never get it to look seamless.  So either use one of the methods above, or strip the whole floor.