Adebrophy - hmmm finding a room with a window in a Japanese office might be difficult!
Ah - good point! Two umbrellas two flashes is the only way. Or three if you want to light the background or create a nice rim light. But the point of using window/ambient is that doing lots of headshots when you need to move around an office a lot and don't get the luxury of setting up with lots of time means that - as you suggested - simpler can be best. It also helps subjects relax more.
One good tip from Strobist.com was using coloured gels to warm up your flash much more than the ambient. Colour correcting afterwards in post makes the non lit sections cooler than the subject. Its a good way of making a dull office look good without having to light the room as well. Another example of more with less that's faster.
By the way, I should add that I'm still quite new to doing paid jobs so unlike some of the others here - who are offering excellent and very valid advice on much more sophisticated lighting set ups - I am sticking to simpler set ups to make things faster and to let me focus on the subject and the shot and not the lighting.
Having said that, as I've grown beyond those simple techniques, I still find that for the most part doing photos of people in at work means speed is more often more important. Execs too busy to stick to time slots, impromptu set ups in meeting rooms, subjects not willing to all come to one location on site... Often keeping mobile and simple is better for the client too, which is another reason a bag of flashguns isn't neccessarily a bad option. If you can stay on top of the batteries!