I'm a very fast sprinter. will 2 hamstring tears in a row affect my speed in the future?

It is best to appropriately warm up and prevent future tears however if you allow appropriate resting and healing it may take a while for you future speed to be affected but it would eventually happen as you get older.

I would not concern myself with the possibilities in the future. You should strive for your goal and if you fall short that is ok. You know you tried your best. You cannot expect more than that. I have feeded professional in college athletes. They strive for their being able to do their best. You should develop a stretching and strengthening program to prevent tears. You are still growing