That made me curious about the the big two appearing as major characters. It clearly wouldn't be a violation, but I hadn't seen it. So I went looking:
There's a character called "Ispep" here. So, that's been done.
Apparently Pepsi's been done as a character, too - and more than once! It happened in a book called Bones of the Moon. (Which is apparently a good read - someone was looking to reread it.)
By night, in dreams, she begins to visit a strange land called Rondua, where the sea is full of fish with mysterious names, where she and a huge, behatted dog escort a young boy named Pepsi across places such as the Northern Stroke, the Mountains of Coin and Brick, the Plain of Forgotten Machines.
Couldn't find one for Coca-Cola, but in The Genius Files:
Coke and Pepsi McDonald are a perfectly ordinary twin brother and sister on a cross-country RV trip with their parents. Except for one thing--they are also being chased by a bunch of psychos who are trying to kill them as they stop along the way at The National Yo Yo Museum, The Spam Museum, and various other offbeat tourist attractions.
(WTG Dan Gutman!!! That sounds like a great read!)
I've never read any of these - just hunted out of curiosity.