Well, at the college where I did both MS Stats and MS Financial Mathematics (simultaneously!), the way that the tuition works, one only pays for four courses each semester -- so the fifth (and the sixth, for those of us who are totally crazy) are FREE. Of course, this arrangement only applies to in-state residents such as myself -- foreigners and people from other states don't get this benefit.
Now, the fact that I did the above while also holding a full-time job, THAT was certifiable lunacy...
Yes it is the same at RU. As you suggest, I was considering the case where someone would not want to take 5 or 6 classes. But you are right to point out that this is technically possible and a way to avoid marginal tuition cost.
Sounds like you did school and work both at a full-time pace toward two degrees. That's very impressive.