How do two Monosaccharides bond together to make one disaccharide?

Through glycosidic bond.

Explanation:

Disaccharide:

Disaccharide is simple polysaccharide. It is formed when two monosaccharides combine together. The monosaccharides are combine through dehydration reaction as a result glycosidic bond is formed between two monosccharides with removal of water molecule.

The reaction is also called condensation reaction.

There are two types of disaccharides:

Reducing disaccharides

These are hemiacetal and have reactive carbonyl group. They can be oxidized to other products.

Non - reducing disaccharides

These are acetals and can not be easily oxidized like reducing disaccharides.

Glycosidic bond:

It is covalent bond formed between the carbohydrate and another molecule such as monosaccharide.

This bond can be alpha or beta glycosidic bond.