You absolutely should call to ask what the objectives of the second interview will be; it demonstrates interest, commitment, planning and preparation. You can only win by calling to ask what to expect, what to plan for etc. If they say it's a surprise then so be it, but it's very unlikely.
If you've covered the technical capability part of the interview, then they might be looking in the second interview to assess the 'behavioural capability' stuff. For a technical person, I know this is hard to give credit to (you guys are always referring to it as the 'soft fluffy stuff' ;o) but it's likely to be important if they are going to explore it. It could cover things like, how you manage your time, how you engage others/work as a team member in order to get things done, how you might approach a conflict type situation with a colleague etc.
Think about the role and what else that role will need you to be good at apart from the coding aspects. The world is full of people who can write good code, do technical analysis etc, so it's the other aspects of 'how' you go about your work that is going to differentiate you from the competition.
If you find out that you have to under go psychometrics, let me know as I may be able to give you more insight into what to expect with those.