I would be looking into finding a less expensive place to keep horse #2 (the sound one) and see if your parents will help you with horse #1. Especially if you could keep them at the same place.
Many barns have some flexibility in allowing boarders to work off some board, usually by doing late night feedings or cleaning stalls on weekends. This is how I’ve afforded my horse through high school, college, and beyond. Maybe you do some extra work on your school breaks and summers to pay more for both horses, and your parents help you out during the school year.
I’ve had 2 horses in a situation similar to yours - my first horse had to be retired, and I was full leasing another horse to ride (who was then given to me). I kept the retired horse a 1/2 mile away from the riding horse at a quiet backyard barn for relatively cheap. Because it was nearly next door to my riding horse, I could be there all the time. My parents paid board on my riding horse, I paid everything else for both. It was hard. I was in law school at the time, so I had more flexibility in my schedule, and I made sure I fit in as much barn work as I could, including cleaning stalls before classes, and going back to the barn in the evenings after classes to ride and work some more. It really was killing me financially, even with how much I was working. My retired horse ended up with a traumatic injury (complete tear of his DDFT, as a complication of his prior treatments for navicular disease) after less than a year in this situation, and could not be rehabbed (we tried). So while putting him down was heart-wrenching, it also was a relief financially. I may not have had a choice in putting him down, but it ended the same as other folks here have pointed out.