Unless I’m misunderstanding, it sounds like in essence what you want to do is use your daughter’s credit to cash flow part of your younger daughter’s education with the promise of paying it back instead of you getting a private loan?
That’s big risk; maybe I’ve experienced too much “life,” but one thing you can rely on is Murphy will be there every step of the way. Keep in mind if something happens, illness, job loss/downsizing, death…your D20 is the one who is on the hook; this is her financial life. One late payment can affect her ability to buy a car or a house at a decent rate; late payments can affect her ability to get a job, as many jobs require credit checks. Is it possible for your younger daughter to find a college in HER budget?