Single Knight MovesWhere a Rook or a Bishop or a Queen moves in straight lines, making their "coverage" long bands of squares along a row or across a diagonal (or, of course, both for the queen), Knights move from point to point, making their "coverage" a set of 8 points as shown in the illustration on the left.

Two Adjacent KnightsWhen you have two knights positioned next to each other, their "coverage" depends on their relative positions. When the two knights are adjacent, their potential moves form two arcing lines, as shown in the illustration to the right. Naturally, if the two knights were next to each other vertically instead of horizontally, the arcs of coverage would be vertical instead of horizontal...

If the two knights are adjacent diagonally, their coverage forms a grid. Since the knight changes color each time it moves, if the knights are
Diagonally Adjacent Knights on light squares (as shown in the illustration to the left), the coverage squares form a grid on the dark squares. If the horizontal adjacent knights cover an arc, more or less similar to a rook, the diagonally adjacent knights form a grid of diagonals, somewhat similar to a bishop.

While the adjacent knight only have two basic positions, mirrored, if the knights move two squares apart, they have three basic patterns (again, mirrored). Perhaps the commonest pattern is the one where the two knights cover each other.
Two Linked Knights In that pattern (shown on the right) the coverage forms a large circle, with two continuous arcs along the diagonals to either side. If the two knights are being used to herd the emnemy king, those two continuous blocks could be important.

The other two "two-fer" positions also form rings. The horizontal or row position (mirrored in the vertical) position, shown to the left,
Row 2-fer Knights forms three linked rings — actually, hexagons.

The diagonal two-fer (shown to the right)
Diagonal 2-fer Knights forms two concentric rings. The open gridwork of the two knights, in any formation, makes it obvious why it's impossible for the King and two Knights to checkmate the other king — it's not so much that the two knights don't control enough space, as it is that they don't easily form walls to confine the other king.