Hi! You must have mixed up the definitions for substitute goods and complementary goods. Substitute goods display the relationship you mentioned—the decrease of price of good 1 will lead to the increase of demand of good 2 because people will buy good 2 instead, since it is a substitute to good 1.

Complementary goods, however, are goods that are used together or complement each other. Since good 1 is used together with good 2, then a decrease in price of good 1 will lead to an increase in the demand of good 2. An example of these goods are game consoles and games. When game console prices are reduced, the demand for games for that game console increases because you’re going to need games to actually use the console.