What would be best would be to explain precisely why the combo you mention would be more effective for you than what you are currently taking to your doctor. If you have a well founded valid logic behind the reason why you need to change up your ADD meds, odds are your doctor will be receptive to changing your pharmaceutical regime.
If you're just trying to conjure up the best way to get high and don't have any basis to substantiate the specific cocktail you desire... well... you'll see what happens then I guess
I find most doctors (psychs anyway, gp's can be a little less receptive and a little more paranoid that you are a crackhead) are very willing to take your thoughts and concerns regarding ADD medications under serious consideration.
If you have a legitimate reason why you think one med will work better than another or why a combination of meds would benefit you, explain it to your doctor. Odds are, if it really is reasonable, he'll give it a whirl.