Have two blogs of your own or you want someone else to work on a blog with you as a team?
Log into your blog. We'll call this Blog1.
Go to the "Setting" tab found under "Customize"
Next go to "Add Authors" and send an invite to Blog2, this is the one you want to add.
If this is your blog you will have to log out now and log in as Blog2.
Accept the invite then return to Blog1.
Return to "settings" and select the "Permissions" tab. From here use the "Grant Admin Privileges" and your all done.