I would suggest starting with a low dosage, my pdoc wanted to start me on 10mg twice daily, but suggested I start on 5mg, so that's what I do and have been doing lol. Of course if they prescribe you a 20mg XR dose, you might not be able to split the dose.
This way if tolerance somehow does build up, have more space to work with, (ie. I can increase to 10mg twice a day, instead of having to increase to 20mg twice I day if I had been taking the 10mg twice daily). Plus some people find that if the dosage is too high, they'll suffer side effects like increased anxiety & depression, I think it's better to see how your body reacts to small dosages first.
But if you do somehow get both Memantine and Adderall, be sure to keep us updated through a thread or blog so we can know helpful it is. Note that some people have found that Memantine had some side effects they didn't think was worth it or didn't find it helping with stimulant tolerance.
Also, if Memantine does help you with tolerance, not sure if it keeps the euphoria feeling for everyone, I think some people aren't seeking this feeling and are just taking it to keep on a lower dose of adderall and to stop increasing constantly to be able to focus & concentrate.
But like said above, please keep us updated on your progress!
Disclaimer: I am not a doctor and am stating the above with information I have from personal experience and what I have read from others' personal experiences.