I have had shared dreams before, but they have only been spontaneous. I don't know of any method except becoming lucid and then looking for the person. Perhaps if two people plan to have the experience, there is more probability that they both remember the experience.
Ps. IMO all dreams occur in the astral realm, just in different levels (I don't mean physical levels, perhaps vibrational levels or levels of consciousness.) So I think the astral realm is constantly influenced by all dreams, not just lucid ones- i.e. it being the collective unconscious.
Cheers,
C.