The symptoms you describe are rather non-specific. There are a number of possible problems that could cause them. Calcium deficiency is not likely to cause the disorientation and falling off the perch in an adult unless there has been on-going chronic, debilitsting disease present.
How was the "respiratory infection" diagnosed? By culture of a bacteria or fungus? By symptoms alone? The respiratory component may be secondary to another primary problem, so antibiotics are appropriate for now, but the other symptoms are not fitting with a bacterial infection.
Vets may speculate if there are no diagnostics available, like bloodwork or x-rays. While waiting for results of bloodwork, cultures, or other tests, it is appropriate to develop a "differential diagnosis", a list of possibilities, and treat for the most likely until results of tests confirm the diagnosis or suggest another one.
If there has been stress from the addition of the D2, possible introduction of infectious disease by the D2, and/or exposure to the dust and other airborne toxins and/or irritants from a recent renovation (for example), there are several possibilities.
1. The D2 may have been carrying an infectious disease, like psittacosis. This needs to be ruled out with testing (both of the D2 and the RB2).
2. The RB2 may have been hiding a low-grade chronic disease that has been forced to the surface by the stress.
3. The dust of the renovations may contain heavy metals or other toxins.
The symptoms you describe are very compatible with heavy metal toxicity. Other possibilities that could produce the symptoms are liver failure, psittacosis, infection of the central nervous system, egg binding, egg peritonitis, aspergillosis, toxins (inhaled or ingested), and PDD. A respiratory infection is not likely to produce the symptoms you describe unless the infection has become systemic and the bird is septic, with bacteria circulating in the blood and affecting liver and/or central nervous system.
I highly recommend that the bird be x-rayed immediately. A bacterial and fungal culture should be done. Bloodwork, including a CBC, chemistry panel, protein electrophoresis, lead, zinc, aspergillus titer, and bile acids should be done. If the vet just wants to keep treating based on speculation and won't (or can't) run these diagnostic tests, then you need to find one that will (or can). Fast.
If you need help in locating a vet that can do these things, let me know by PM'ing me with your location and zip code and I will try to find someone to help.