Can a mixture produced from two already patented chemical compounds be patented?

It depends. This depends if the combination is non-obvious. Obviousness is a term of art. Obviousness is very difficult to predict and is one of the requirements for patentability. It is the number one patent killer and the USPTO favorite killing tool. In order to evaluate if the combination of the chemical compounds is not obvious you have to answer the following questions: 1. at the time you made the new composition, was the composition, (and any method of preparation or physical change of the compounds) foreseeable by some one else with your same skill in chemistry? 2. does the compounds in combination perform a similar function that each compound performs individually? 3. if there are similar compositions using the both compounds out in the public, is this new composition, a step forward, a far better composition? 4. did you solve a problem not solved before by mixing the two compounds? 5. was the result of mixing the compounds in this particular way unexpected? 4. did you manipulate the physical properties of the compounds? 5. did you use a special method (technique) of preparation of the compounds, for example did you use temperature and pressure, or did you bake the compounds before mixing? 6. was the result of mixing the compounds predictable? 7. did other chemists try to solve the problem but failed and you and your inventive mixing method succeeded? Only after answering these questions a patent attorney can determine if your invention is patentable. Another issue that you will face is the fact that the two compounds are patented. Even if you get a patent on the mixed compounds, you have to pay royalties to the owners. Now, if you get the patent for the combination, you might enter into a cross-license agreement if the owners are interested in you inventive combination.