Maybe it's a bug, maybe they don't think you should have more than one toon in a fleet? Either way, having your Fed fleet you made and your Fed aligned Romulan unable to join the same fleet is annoying, to me at least.
Here's what I did
Drop your toon out of the fleet, have someone send you an invite back, don't accept it!
Log out and into your Fed Romulan and they should be able to send you an invite as well, accept it then log back into your Fed toon and the invite should still be there for you to accept! Click it and join!
Now both characters are in the same fleet.
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