As noted several times in previous replies, there's no good reason to use interrupts for the push buttons in this application. A properly-written, non-blocking loop() function will do the trick.
Benni_Lorenzo:
Sorry, i cut something off there. What do you mean atomic?
In this context an atomic operation is per the definition here:
It executes as a single, indivisible operation. Nothing else can happen while it's executing. On an 8-bit machine, accesses to 16 and 32 bit values may not be atomic. Any interrupt and associated ISR could fire while it's in progress. If that ISR changes the value you're trying to access part way through, you will get unpredictable results. Since you're running on a 32-bit machine (Due), the risk is less. Still, I'd protect any accesses to shared (volatile) variables in non-ISR code with noInterrupt() / interrupt() pairs.