I got round this by lopping a month off the Current membership status rule and creating a new status rule Reminders which is the end date minus 1 month. I then created a smart group with primary member set to no, and the Reminder status. You end up with a group of individuals who need a reminder sending 1 month before membership expires. I then manually send out the reminder to the group with the checksum link.

I got round this by creating a group for the non-primary memberships. I then set up a membership scheduled reminder and limited the emails to the group I created.