I guess it depends on what you see as a romantic relationship.
in "Character Strengths and Virtues", Peterson and Seligman say something like this:
“love is marked by the sharing of aid, comfort and acceptance.”
comfort and acceptance might be very high in an identity relationship too BUT a dual has the best chance at helping you how you want to be helped.
If you enjoy your dual only while he or she is away... you might be leading a very unaware life.
Paying more attention to the present moment will reveal the true effect of your dual.
Also, it is worth mentioning that paying attention to the present moment is something that can be developed with proper practice, just like a muscle. That practice is meditation.