The principle of two weaknesses is (I assume) fairly well known. Normally a defender has enough to hold one weak point in a position, but protecting 2 or more is often too much. For attacking players, the principle of two targets is a similar idea. If you have at least two distinct targets in a position, then you can be pretty sure your attack is going swimmingly.
The following game was played at my club last week. Black has made the cardinal sin of playing passively in the opening, and as a result White has a lot more space to work with. In the diagrammed position White can target either the rook on a8 (with Bf3) or the King, with Nxh7. White found the mating attack a more attractive option, and after the sac on h7, Black's resistance crumbled.
1.d4 Nf6 2.c4 e6 3.Nc3 Be7 4.e4 d6 5.Nf3 0-0 6.Be2 b6 7.0-0 Bb7 8.d5 c6 9.dxe6 fxe6 10.Nd4 Bc8 11.f4 c5 12.Nf3 Bb7 13.Ng5 Bc8 14.e5 Ne8 (D)
15.Nxh7 Kxh7 16.Bd3+ g6 17.Qh5+ Kg8 18.Qxg6+ Ng7 19.Qh7+ 1-0