My husband and I have BB 's and we had to EACH get a data plan....oh well, I still LOVE my Tour though (1st BB that I ever had) and when....I start working at the new job, will get a discount on plan, so that should help with the price!!!
As already mentioned you need a separate data plan for each BlackBerry. You can share voice minutes though. I have a plan and pay the $9.99 / month to add a line for my wife to share the minutes.
Discounts are also applied "intelligently". I get a 20% discount through work but the discount is only applied to the voice plan (since I am the primary), my data plan, and accessories that I buy. The discount is not applied to the monthly add a line fee, services for her phone (such as the VZW navigator), or any accessories she purchases.