Are you referring to Synthetine and Syntheselen from Synthetek? If so, I take both 30min's prior to cardio in the a.m. and again 30min's prior to weight training later in the day.
Ideally 30-60 minutes prior to activity weather is strength training or cardio. Synthetine helps release fatty acids into the bloos stream so they can be burned as fuel. Syntheselen helps your body to create more energy for your training. Combining the two together prior to training makes an optimal training session.