The prophecy of the separating of the Wheat and the Tares is the first of two end times judgments where there is a separation, and a dividing of the saints from the unsaved.  However, this prophecy is distinct from the Sheep and Goats judgment in the fact that it occurs prior to the rapture, during the Pre-Tribulation Revival and Harvest.  And, like a harvest, its characteristics are different as well.  In the same frame of reference as the Sheep and Goats judgment, the Wheat in this prophecy are the saved, and the Saints of God.  The Tares are those who are of the Devil (witches, the evil unsaved, fake saints, etc) and lurk within the body of Christ, pretending to be one of them, but are the complete polar opposite, even though they do a good job of playing the part.

What makes this a prophecy all its own, separate from the Sheep and Goats judgment, is that a field is involved in this, and a harvest, where the Tares are separated out and sent away to be burned (removed, separated, burned, sent to Hell) and the Wheat is gathered into bundles, which will later be removed from the field (the rapture).  The Sheep, on the other hand, are not.  They are left in the field, pointing to the soon start of the Millennial Kingdom shortly afterwards.  This prophecy also further distinguishes itself from the Sheep and Goat prophecy by the fact that there is a harvest field (the world), but no such field in the second, as the harvest has long since been completed by the time of the second prophecy, leaving nothing more than a separating of the good and evil, allowing only those who love God to enter the kingdom.

It is also distinct from the Sheep and Goats judgment as the verses indicate a time period over which this occurs (ie, it's not all at once, but occurs over a period of time) vs an event that unfolds all within a few hours, to a few days at most.  The Sheep and Goats judgment also occurs at a distinct time in history, whereas the Wheat and Tares prophecy has no specific date or time period set for it to occur.  It only references a time period, but not anything as specific as the Sheep and Goats judgment has.  It is also divided out in scripture in two different sets of verses, relayed back to back, which indicate two separate events, rather than just one in two forms, or described two different ways.