How long would two 10mg IR Adderall last?

In the morning I take a 10mg IR pill at around 7:30 and it fades around 3:00 PM. When I take a 20mg XR at the same time in the morning, it lasts me until 12 AM and cant sleep. So Im trying to find the right dose the would last me until around 8-10PM. So how long would two 10mg IR last? Thanks