To determine how many molecules of water would be used to form two glucose molecules via photosynthesis, we need to examine the chemical reaction for photosynthesis:

6CO₂ + 6H₂O → C₆H₁₂O_{6 (glucose)} + 6O₂

This equation shows that for every six molecules of carbon dioxide (CO₂) and six molecules of water (H₂O) used in photosynthesis, one molecule of glucose (C₆H₁₂O₆) is produced, along with six molecules of oxygen (O₂) as a byproduct.

Given that we want to form two glucose molecules, we can simply multiply the coefficients of the reactants and products in the balanced equation by 2:

12CO₂ + 12H₂O → 2C₆H₁₂O₆ + 12O₂

Therefore, to form two molecules of glucose, 12 molecules of water (12H₂O) would be required as reactants in the process of photosynthesis.