Actually, it would be possible for them to meet the future Marty and Jennifer, since from the future time point, Marty and Jen have already arrived back in 1985, thus completing the circuit, and just leaving the younger time travelling selves the opportunity to come forward and visit them without appearing to have disappeared.
As Captain Janeway said once, time travel gives her (and us) headaches.
It's the Bill & Ted rules of time travel. "When we get back we'll put (whatever) right here so we can use it now," and there it is.
Marty was only gone from 1985 if he didn't go back to the same time he left.