It's called their "sum".
Two numbers added together to form a sum are called "addends".
The numbers in an addition sum that are added are called the addends.
Assuming it is an addition sum - in any addition sum such as 5 + 6 = 11, the numbers 5 and 6 are called addends.
When adding numbers, the total is the sum. The sum of two odd numbers is even.
The sum of two odd numbers is an even number.
Combining two or more numbers to find the sum is called addition.