Before anything else, it is worth noting that combining Modavinil with amphetamine-based drugs such as Adderall can be risky. Amphetamines are stimulants, and Modavinil produces stimulant-like effects too. Consequently, taking them concomitantly can lead to overstimulation, which in turn increases the risk of experiencing side effects such as severe headache, nausea, or anxiety.
Although doctors may sometimes prescribe both drugs together, it is important to strictly adhere to their instructions to reduce the risk of adverse effects. In a nutshell, do not mix both drugs!