Mercedes does not call it a PCV valve; However, the crankcase rebreather valve should be changed at normal maintenance intervals and if you are a DIY type may overlook this. It can cause your air intake manifold to get clogged internally which will lead to broken plastic (externally/internally).

Watch how easy it is to replace in this video:

Do we need two of those ?

Nope. There is only one.

Where do I find info on how to replace the Pcv on my e350

You are on the wrong forum. Please post the year of of your E350.

Did mine a few weeks ago, knew it needed it as soon as I smelt burning oil.
When they leak they drip right onto the hot cat.