Is it against google TOS to have two ad units on the same page? I have read some other threads where they say that it is. But then, how come there are so many sites with at least two adunits? I am a little confused
You can only have 1 Link Unit and up to 3 regular banner units. I believe if you add more then allowed amounts it just shows blank. For information on adformats and sizes visit: https://www.google.com/adsense/adformats