Orange does indeed offer BIS on pay as you go. You NEED that for blackberry mail and Blackberry Messenger you're interested in. It only covers network access that goes over BIS. If you browse with a third party browser, use applications that drop transport outside BIS (direct tcp/wap etc) they charge you by the kb. You can get the internet bolt-on (or whatever it's called this week) by paying extra and you can have that use inclusive.
Browsing on the blackberry with the internet plan sim from the other phone would work if you setup your APN settings. Blackberry Email and pin messaging will not. You need the BIS Data Plan for those.