And conveniently I can make the basic N moves which don't block the position whilst black takes back the final three N moves.
From here white uncaptures to leave a black N on b3, three moves back for each side.
This can be reached as the black c or a pawn can be left on the fourth rank after my retractions (to follow) in the first instance. Plently of opportunity to waste time still available.
Therefore total moves:
bN b8-c6-d4-b3 (3 moves)
bN g8-f6-g4-e3xf1-e3-d5 (6 moves)
wN b1-c3-a4 (2 moves)
wN g1-h3-f4-g6xf8-e6-c7-a8 (7 moves)
Total of 18 moves then.
Edit: I can't go via e6 and c7 as I'd check the K on the way. e6-f5-e6 would solve the problem with two extra moves (to make 20)
It is possible for the N to move from f8 to a4 and the other from b2 to a8 but these both take the same number of moves.
My answer is twenty, not 18.