What the app description says, literally, is this: ★ Share tasks with your business partners by using the same Gmail account. The text is the same for both Pro and Std. And so far it's only way to share tasks between two people, regardless of which version you buy. I will add multiple account support to Pro at some point, so it should get easier, but please do not make a purchase based on a promise.
The easiest thing for you to do right now is to create a special Gmail account moc.liamg|efiWsiHdnAnhoJ#moc.liamg|efiWsiHdnAnhoJ and use Google Tasks in that account for joint planning. You can then access Google Tasks using Google's web site on most modern smartphones (I'm sure there is at least some support for Blackberry from Google's web site, but you can easily check that). Additionally, you can purchase a version of GeeTasks for iPhone and hook it up with that account as well - the native app is much easier to deal with than the web site. If you want to use more than one Google account with GeeTasks you might want to get two different versions of GeeTasks, such as the free (but limited) GeeTasksLite.