If deleting one of the 2 partners isn't the only thing linking another part of the tree to the founder, when you select "More actions..." there will be an option to delete them above the cancel button.
if you can't delete either, you're going to need to figure out what section of the tree would be sever from being able to be linked back to the founder and either delete that section, or figure out a way to link that section back to the founder that doesn't include the partner.