Ok, say there is one portal currently open to another world. If two investigators have gone through the portal and one is ahead of the other, what happens if the first one to leave closes the portal?
Is the remaining investigator (who now has no portal left to escape the other world) "lost in time and space"?
Depends. First, you do not deal with the 2nd investigator until he is moved back to Arkham.
If the gate is open, he may return as normal (so the 1st investigator through may choose to wait on closing the gate until he gets through as well). If they arwe both through, they each get an explored marker and may both attempt to close the gate.
If the gate is closed, then it is at that point he would be LiTaS. So the 1st investigator could close the gate and hope that another gate to that world is opened before the 2nd one would have to move.
Does he get to exit the other world through a different open portal?
Only if there is another gate to the same world. (there are 2 of each)
Can two investigators even go through the same portal?