To determine how much extra food is needed to feed the second cat, we need to find the difference between the total amount of food Jerry serves now and the amount he served when he had only one cat.
When Jerry had one cat, he served:
\[ \frac{1}{4} , \text{can} \]
Now that he has two cats, he serves:
\[ \frac{2}{3} , \text{can} \]
To find out how much extra food is needed to accommodate the second cat, we subtract the amount served for one cat from the total amount served for two cats:
\[ \text{Extra food needed} = \frac{2}{3} - \frac{1}{4} \]
To perform this subtraction, we first need a common denominator. The least common multiple of 3 and 4 is 12. We will convert both fractions to have a denominator of 12.
For \(\frac{2}{3}\):
\[ \frac{2}{3} = \frac{2 \times 4}{3 \times 4} = \frac{8}{12} \]
For \(\frac{1}{4}\):
\[ \frac{1}{4} = \frac{1 \times 3}{4 \times 3} = \frac{3}{12} \]
Now we can perform the subtraction:
\[ \frac{8}{12} - \frac{3}{12} = \frac{8 - 3}{12} = \frac{5}{12} \]
Thus, the extra food needed to feed the second cat is:
\[ \boxed{\frac{5}{12}} , \text{cans} \]