What would happen if you have a double nationality let's say Mexican American and both the US and Mexico got to war and a draft is needed what would happen if both countries draft you?

You would be drafted by the country you are living in. Even if you aren't, you would have a hard time, as for the duration of hostilities, both countries would round-up all enemy countries' nationals to prevent sabotage, spying or terrorism.

That precisely happened to Japanese-Americans who lived in the US during WWII: 110,000 of them were forcefully sent to 'internment camps' for the duration of the war (See Executive Order 9066).