Does your older kid’s school guarantee to meet full need for all accepted students? If not, you might not see one dime in additional need based aid.
When our younger kid started college, our older one was still in college. His EFC per FAFSA was $22,000 on a school which cost $40,000 at the time (2006-2007). He didn’t get a dime of need based aid (he hadn’t the first three years because our EFC exceeded the COA)…but his merit aid increased by…$250.
So…just apply for the aid and see. You can do the net price calculator for your high school student and see what your net cost will be with two in college for him.