Only’s and Middles make a good match because Only's like to take charge and Middle's are only too happy to have someone else take the reigns.

Middles are oriented towards compromise and pleasing others while Only’s (who were the sole recipients of their parent's attention) tend to enjoy being pleased.  Middle’s are generally not emotionally dramatic while Only’s tend to be emotionally self-absorbed. 

For long-term success, Only’s need to realize they are not the center of the universe and constantly reassure their Middle that they are loved.  Middles need to make efforts to share their feelings before their pent-up emotions become a problem.