If both have been fed for at least 2 weeks and the horse is not hypersensitive to feed changes, you can switch between the two products. That's how I do it. I have both in the stable and when it's cold and/or wet (the Iberians don't like that that much) I give them the winter starch and when it's warm days again I give them the metabolism booster. This works great for my two and I can optimally support them holistically, depending on the weather. But of course you can also give the booster in autumn, then 1 to 2 kg of winter starch for the coldest time and then the metabolism booster again.