First, I would make sure that the doctor knows that he is on Ami, before starting the other, as they may have some effects that you don't want.
However, if it is the doctor's intentions to have him taking both, which it is very possible that it is the case. My doctor always has me on what she likes to call a "cocktail" and I don't mean a mixed drink ha ha, I am generally on two different preventatives at a time and then two types of abortives at once (never triptans at the same time, and always things that the doctor and pharmacist agree are compatible)
I have taken antidepressants and beta blockers/calcium channel blockers, topamax, all kinds of things in different combinations in efforts to keep the migraines at bay and I figure whatever works and does't harm me...I am all for it!
Right now I am taking 400mg of Topamax daily, as well as a blood pressure medication that is used to prevent migraines. When I take abortives, I use DHE injections in conjunction with Zofran for nausea.
I have much better luck with combinations of medication than one alone.
Just a thought!