The synthesis reaction that connects two monosaccharides to form a disaccharide is called a dehydration synthesis. It is not a synthesis of the type A + B → C.
In a dehydration synthesis, an H atom is removed from one monosaccharide and an OH is removed from the other monosaccharide. The fragments combine to form a disaccharide and a water molecule.
An example is the reaction of two glucose molecules to form maltose.