Some people do take two doses of a long-acting stimulant. With Concerta, the lowest dose capsule is 18 mg, and it cannot be split or it won’t be long-acting anymore. It’s possible you could switch to short-acting stimulant medication and take it when needed. Definitely talk with your doctor about the options.
It’s possible that something might be negatively affecting your medication, like another medication you take or even drinking something with citric acid, like orange juice. It’s also possible that a protein boost just prior to that afternoon mental fog could eliminate that side effect.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism