Combine stamps for correct postage?

Hello, I want to send a large letter, but don't want to go to the post office to have it franked. The postage would be €1.60, but I only have 85 stamps left. If I stick two of them on the letter, it would cost €1.70. That would be 10 cents too much, but I really wouldn't care in this case. My question is, is this even allowed? Can I do this, or will the letter be returned to me?

Thank you in advance