I’d say treat them to lunch separately, get an idea of where they want to be in five years and try to impart some wisdom on how to get there. Try to make it known that you aren’t asking this question from the perspective of your company but as a mentor type figure. Be honest about how they can accomplish those goals.
Here’s where it gets different for each candidate.
For the more stellar intern, if the company can offer what they need to attain their goals, mention again that you’re trying to get her on the team full-time and want to make sure what they offer is fair. If her goals don’t really fit the company, mention that you are more than willing to give a generous recommendation if needed but also mention that an offer will probably be made and if she were to change her mind, an idea of what she thinks is a fair offer would be helpful.
For the other candidate, I would suggest being understanding about how tough the potential rejection is going to be. If you can find a way to give advice that doesn’t seem belittling, it might be helpful to them. It could help them become a stellar engineer rather than just competent. Mention that if their plans are outside your company, you would be willing to give them a glowing recommendation, but also mention that they will likely be receiving an offer but you’re not entirely sure what range the offer will be. If they give what they think is a fair range, mention that you’ll try to see what you can do.
In the mean time, if you want to be proactive, look at both candidates’ education, gauge what their student loans might be, see what other companies in your industry offer for entry level engineers. Try to guess what their budget might look like and see what they might expect as an offer from another company and make your suggest based on what you find.
As far as how you approach management, mention that the first candidate is a “need” and the second is a “want.” If you can get both, that’s great, but you are focused on the first. If the second candidate does not get an offer, give them your condolences, write a recommendation letter, mention again that you would gladly be a reference and maybe make some suggestions on where they could look. Also, if the second candidate doesn’t get an offer, maybe try keeping them on as an intern. It’s typically easier to find a job when you have one.