The most common form of double attack is the fork, a simultaneous attack by one unit against two hostile units. The aggressor unit inserts itself at some midpoint, between the two attacked units, and the defender finds, to his chagrin, that he can save one of his men, but not both.
In theory, any chess piece has the capability of forking two enemy pieces. Knight fork and Queen Fork occurs with most frequency, then come bishop, rooks and pawn forks. A fork by the king is rarely seen.
The arrangement of positions is according to the pieces that is doing the forking. Knight forks are trickiest, so they get most attention.