When asked to attend to the difference, the student immediately can differentiate them. We work together until we have at least a couple of notes strung together legato. Sometimes the student can do two sets of fingers. At the next lesson, we try threesies. Depending on this how goes, we'll do threesies at home the following week or maybe move on to foursies or even fivesies.
Hands are still apart.
I discuss it with the child, who sees right away that those guys need some extra work, so starting with them will allow them all to end up even in the end. We work through twos, threes, fours, and fives, as described above. Meanwhile, in the songs, I point out the need for hod fudge in the places where the student has demonstrated by the hot fudge exercise that he can play legato. This is a song that has been passed off for at least a month. After this regimen, occasionally students will need a reminder, but a verbal hit is usually enough.