Will the 2 liter fit in the side bottle pockets on a backpack?

It all depends on what pack & side pocket you're talking about. I carry the two liter in my side pocket & not filling it all the way, then using the lower side compression strap to secure the top of the Platy. When it contains less water, I then fold it in half & stuff the entire bottle in the pocket. Pictures to follow.

Not too likely. Most packs with functional side water bottle pockets are designed to snugly hold a 1L Nalgene, which is 3.5". The 2L version of this is over twice that diameter.