I have ADD as well and take meds. If there is one thing I can tell you it's that you should never double up on your meds.
Not only does it just not work any better at all, but due to a mistake in my pharmacy I accidentally took double what I was supposed to use for almost a year. This resulted in me being moody, easily agitated and in a constant state of hyperfocus.
By the time my doctor finally found out what was going wrong, I had lost my job, ruined my relationship and I was severely depressed.
She said that if she hadn't noticed that I could had eventually developed psychotic episodes and/or other mental issues.
This was a year ago now and I'm much better. Fortunately got back together with my girl.
TL;DR: It's never a good idea to double up on meds.