Does anyone know of any problems in which a KNN-K mate is forced in the end? It is fine if you make one yourself.
Here's a #4 where White has just KNN and there are no Black units blocking Black king flights:
[Title "C. Barton; Family Herald 5 Nov 1859, no. 65"] [StartFlipped "0"] [fen "8/8/8/8/8/6K1/4p1N1/4N1k1 w - - 0 1"] 1.Ne3 Kh1 2.Ng4 Kg1 3.Nf3+ Kh1 (3... Kf1 4.Ne3#/Nh2#) 4.Nf2#
Here's a #5 where White has just KNN and the play is a bit more interesting than in Barton's #4:
[Title "Otto Dehler; Denken und Raten 31 Dec 1934"] [StartFlipped "0"] [fen "8/1p6/8/8/3K4/N7/8/k1N5 w - - 0 1"] 1.Kc3 b6 (1...b5 2.Kc2 b4 3.Kb3 bxa3 4.Kc2 a2 5.Nb3#) 2.Kc2 b5 3.Nb1 b4 4.Nd2 b3+ 5.Ndxb3#
To get the following position as a checkmate, the last move must be Nb3#.
[fen "8/8/8/8/8/1NN5/2K5/k7 b - - 0 2"]
Prior to Nb3#, black must have made a non-king move, so must have moved something to b3. If it were anything other than a pawn, it could choose to avoid the b3 square, so it's not a forced win for white. If it were a pawn on b4, it could capture the c3 knight, which is also not a forced win. Thus black must have had a pawn on c4, and be forced to capture on b3 on black's previous move.
So here's one such puzzle (LiChess):
[fen "8/8/8/8/2pN4/2N5/1PK5/k7 w - - 0 1"]
White to move and mate in 2.
It's possible to move black's pawn back a bit, and give a slightly longer puzzle with the same idea:
[fen "8/8/2p5/8/7N/2N5/1PK5/k7 w - - 0 1"]
White to move and mate in 4.
I haven't exhaustively checked all possible flips and rotations of the board (which makes more difference than I initially thought). There's also a few possible mating patterns, but I expect the idea is going to be much the same in each case.
This is a real puzzle solving interface for one of the mate in 4 ideas given in another answer. There are many alternative solutions ending in 4.Nb3#
The mate in 4 from another answer can be presented as an interactive puzzle with this link https://www.apronus.com/chess/puzzle/?p=08X8X8X8X8X6K1X4p1N1X4N1k1_w_-_-_0_1&N=4&w=uqtqowd-uqtqoqp-uqtqoqf&h=%3Ch1%3EMate%20in%204%3C%2Fh1%3EC.%20Barton%3B%20Family%20Herald%205%20Nov%201859%2C%20no.%2065&m=9
While the mate in 5 is this https://www.apronus.com/chess/puzzle/?p=08X1p6X8X8X3K4XN7X8Xk1N5_w_-_-_0_1&N=0&w=owaqeqiqw-oqsqaqoqw&h=%3Ch1%3EMate%20in%205%3C%2Fh1%3EOtto%20Dehler%3B%20Denken%20und%20Raten%2031%20Dec%201934&m=9