Of course it's appropriate. Some companies even give you documents on how to prepare for their interviews, because they would like to interview the candidates who are best prepared.
You should ask for information on the process or how to best prepare for the interview. Things you may want to learn:
- number of interviewers
- materials available (open internet, whiteboard, IDE...)
- length of each interview
- types of technical questions covered (coding, architecting, business)
- technical topics covered (sorting, searching, on and on)
Things you will not want to learn:
- "Should I be on time or try to be early?"
- "Do you have any tips?"
- "Are we going to be talking about salary today?"
- "Will there be soft, HR-type questions?" (every conversation is one. Don't imply you need index cards to talk about your greatest weakness.)
- "How should I dress?" (if you've never been to the company, maybe ok.)
It's nonsense to suggest that this is unfair. (Reminder: Nothing in life is fair.) You should not be giving them instruction on what is and isn't fair. Excluding race, gender and a few other things, the hiring manager is the sole decider of what is fair and will tell you what s/he feels is appropriate. You should not worry about what you will learn and the other temp won't. This is also a good time to remind that jobs are won on network and especially as an engineer it's better to stop seeing this as incongruous with proper job-landing sooner than later.
Your interviewer and you have a mutual interest in you giving the best interview possible. This should be your framing when asking these questions. e.g., asking for "tips" is merely going to say "so are there any mistakes in your interview and how do I skirt around them?" This is something you ask someone else who had to deal with their snobbish, gnarly interviewer; not the interviewer him/herself.
For the record, if I were in your position, and assuming that I'm secure about my job prospects either way, I would freely discuss the interview with the other temp. One, it increase both of your chances of getting the job at all. Two, it's just good business. It's a stronger connection you'll have going forward, and better to form a strong connection and miss certain opportunities (decide whether this is one) than form a dubious one.