I think your best bet is going to be making a new calendar (I think you can do it with your standard gmail one too, but starting a new one would probably be best if you don't have too much to lose) that the two of you are going to share. When one of you makes the new calendar click on the down arrow located to the right side of the calendar under "My calendars" and then choose "share this calendar." From here you want to enter the email address of the people you want to share with and under permission settings choose "make changes and manage sharing" so that they're able to edit it. That should do it. Just make sure you set your phone to show this new calendar.
If you're interested in using Thunderbird for Google Calendar as well, check out my tutorial posting events to Google Calendar with Thunderbird
here. The instructions require Thunderbird version 5+ and are for Linux but it should be roughly the same regardless of OS.