The sum can be $4,6,8,12$. Some casework:
4 can be written as $2+2, 1+3, 3+1$
6 can be written as $1+5, 5+1, 4+2, 2+4, 3+3$
8 can be written as $2+6, 6+2, 3+5, 5+3, 4+4$
12 can be written as $6+6$
The total number of ways is $14$. There are $36$ possibilities. The answer is $\frac{14}{36}=\frac{7}{18}$.