I don't think you should recommend that people just try amphetamine or cannabis, much less in tandem. There are serious side effects especially if you have an existing or latent mental illness. Cannabis can induce a psychotic episode in a nontrivial proportion of the population. Amphetamine can, among other things, induce mania in someone who has bipolar disorder. It's fine if you know the risks and want to do it anyways, but Jesus Christ, I don't think you should tell people to use speed because "it helps them concentrate".