I think two People can be so into each other , that the dreams they have may almost seem like they coincide and that has happened to me before.
For a fact : I know when Dark Magic is involved , that two People can be forced to dream or have a Nightmare , that is Identical down to the fine details. *shudder*
I know this because it happened to me and an Ex , when we ran across a Cult making some kind of Sacrifice.