The carb content will be somewhere between the two. It is not quite as easy as averaging the two percentages, because the cans likely have differing amounts of calories. So, the way to determine the % calories from carbs for a mix of two foods is:
% carbs in food A x calories in food A = calories from carbs in food A
% carbs in food B x calories in food B = calories from carbs in food B
% carbs of mix = (calories from carbs in food A + calories from carbs in food B) / (calories in food A + calories in food B) X 100
For example, if you mix a can of food that is 4.5% carbs and 100 calories with a can of food that is 9% carbs and 80 calories:
4.5% x 100 calories = 4.5 calories from carbs
9% x 80 calories = 7.2 calories from carbs
(4.5 + 7.2) / (100 + 80) x 100 = 11.7/180 x 100 = 6.5% carbs
Second example, if you mix a can of food that is 0% carbs and 120 calories with a can of food that is 9% carbs and 80 calories:
0% x 120 = 0 calories from carbs
9% x 80 calories = 7.2 calories from carbs
(0 + 7.2) / (120 + 80) x 100 = 7.2/200 x 100 = 3.6% carbs