I wouldn't ask at the first interview if I were you and I have hired >100 people in my time.
Wait until the second interview or better still wait until you get a job offer. As a graduate recruit they will be investing in you and you also investing some time with them in return so I would consider both jobs much more on what they can offer you as experience, interest and fun over the probably minimal differences in sallary
250 interviewed last week, not all by me of course, all of them on a graduate programme all have the salary explained to them at the start as it's asked about 95% of the time. They sign the form to say they've understood as it has been known for some to say "That's not what I was told." Some ask for it to be explained a little further and what other benefits there maybe, all are very reasonable questions and not even considered against them in the interview.