This is my ideal solution and involves some extras that you may have/may not want... but it's what I'm looking at doing.
I have a NAS attached to my router that has my iTunes library (all music and all movies). Only recently have I removed the music library completely from my MBP's hard drive and now I option + open iTunes to select that library stored on the NAS. From there I sync all of my content (music, tv shows/movies if they're converted properly but usually just music). This sucks because I also stream the video to my TV and need to keep my laptop on and open. This is now where it gets hypothetical..
Have your mac mini running without anything connected to it. Use your MBP to remotely control the mac mini when necessary. On the mac mini keep that iTunes library always open so that the content is available to be streamed. Plug in your iPhone and set-up how you want the music to sync (ideally set it to 'automatically sync when connected' since you won't easily be able to click 'sync' since the mac mini is running without monitor/keyboard). Do the same with your partners. Then you can remotely control that mac mini with your mbp to organize playlists, add new content, etc as well as control what is syncing to your respective iPhones - which would be done automatically when they were connected. That takes care of syncing the music.
As long as the mac mini is running and iTunes is on, you can use the iPhone Remote app to connect and control iTunes. You could install some VNC software to launch iTunes and control your mac mini with the iPhone as well, but if it's always running the Remote app is fine. That takes care of controlling it to be streamed.
For contacts it gets a bit messy (I think) though I'm unsure because I've never actually had to do it. Solution 1 - put both of your contacts on the mac mini and sync them all.. could be a problem if you want to keep the contacts exclusive. Solution 2 - is there an option in iTunes to only sync a certain group of contacts? (again, never had to do it so I don't know). In that case, all contacts could be put on the mac mini and separated in to groups to by sync'd separately. Solution 3 - a MobileMe account used to sync contacts wirelessly from each of your MBP.
Now if you ever wanted to listen to the music on your laptops (while at home) you could turn on the Sharing feature of the mac mini itunes library and then when you opened up your itunes on the MBPs you could connect to it and stream the music - won't work for video though.
I probably rambled a lot there and may have caused more questions as opposed to solving any.. that's just what I plan on doing to centralize my iTunes content but I'm just one person, it could get tricky with two.
