How to display a double digit decimal value on a seven segment display?
You can use a ROM but that's sort of cheating. 

Another method is to detect when the number is >9 and add 0x6 to the number (you need to have another 4-bit adder). Then you will have 0..9 in the least significant digit and 1 or 0 in the most significant digit (if the number before the second adder is > 9 it is 1, otherwise 0).