The equation you’re referring to seems to be part of a larger chemical reaction involving water (H2O). Water itself consists of two hydrogen atoms and one oxygen atom. However, the reference to having 2 oxygens in the product might indicate the reaction of water being split into its constituent elements.
If we were to look at the electrolysis of water, the reaction can be represented as:
2 H2O (l) → 2 H2 (g) + O2 (g)
In this process, two molecules of water (2 H2O) produce two molecules of hydrogen gas (2 H2) and one molecule of oxygen gas (O2). This is where the confusion may arise. The key point is that for every 2 molecules of water, only 1 molecule of oxygen is produced, which corresponds to the 2 oxygen atoms in the O2 molecule.
To summarize, in the decomposition of water, each water molecule contributes to the production of oxygen, and thus, for every two molecules of water, we get one molecule of oxygen gas, leading to the appearance of 2 oxygen atoms when represented as O2.