In the former, the Reels merely overlap and there is no element of sharing of dancers; those who would normally pass By the left in the Middle instead Dance Left hands across halfway with the Middle two dancers in the other Reel as, for example, in bars 17-24 of Bonnie Lass O' Bon Accord where one Reel has its axis Up and down and the other Across the Square set. 8 bars are adequate in a Strathspey but the deviser of the Reel, Angus MacLeod, wisely allocated 12 bars (21-32) for the Interlocking reels of four on the Diagonals in the Longwise set, with 4 bars for the loops at the Ends.
Interlocking at the Ends always involves sharing dancers and is almost certainly exclusive to Crescent reels of four.
It is particularly effective in bars 9-16 of the 3 Couple, Triangular set Strathspey, The Wind on Loch Fyne, where 6 Scottish Country Dancers manage to perform three separate Reels of four, each Man appearing to Reel with the three Ladies and each Lady with the three Men; at the start, each dancer passes Partner By the right, passes the next (actually the 2nd Corner) By the left, Dances all round the next (actually the 1st Corner) By the right, passes the next (2nd Corner again) By the left and passes Partner By the right again back to Places. In Diagram 43, the dancers are shown in their Original Places in the 3-Couple Triangular set at the vertices of the shaded hexagon and Facing in the appropriate direction ready to start.
The arrow heads show the situation after 2 bars, at which time 1st Man is passing 3rd Lady By the left (their path shown emboldened) as are 2nd Man and 1st Lady (their path shown emboldened and broken) and 3rd Man and 2nd Lady.