This is basic, and any Idea user can explain it to me.
I didn't find the answer anywhere, but feel free to help me with a link.

When I chose two different engines (each with one core) to work on a project, how is the actual tree build? Each search indepently without regard to the moves that the other engine has already searched (this is redundant but creates a tree which is double-checked in most of the moves)? Or they only search new territory? Or other algorithm still?

Thank you. This is really bugging me.