How to deform a plane according to the shape of other objects I have 2 planes: 1 with Displace modifier (so random hills and valleys according to a cloud texture) and another flat of different size. My question is if it is possible to deform the flat plane according to the first plane. 

Depending on where I place the second plane regarding the first plane it'll take exactly the same shape the first one has at the position where I place it. Think of the second plane like a piece of cloth that when I put it on top of a rough surface, depending on where in the other surface, it would deform and take the shape of the irregularities it is placed on.
If the base terrain plane is deformed by a displace modifier why not just give the second plane the same modifier stack?

If I understand correctly this is the effect you are aiming for, kind of like a manta-ray wavy hover motion



Just add a second plane above the first one, subdivide it accordingly, and link the modifiers to the original one. Select new plane > select terrain > Ctrl + L Modifiers.

Make sure both objects have the same displace modifier or most importantly share the same displace texture and with the both Texture Coordinates set to Global