:!: Firstly, you will need separate housing for Phoenix right away.
Not only is keeping 2 dragons in the same enclosure irresponsible as they are solitary animals, but if you are sure one is male and one is female, once they hit breeding age they will be trapped and the female will be over-bred.
Not to mention, if there is something wrong with your main guy, adding the stress of a cage mate, along with the possibility of being picked on, or if he has any parasites, etc, that will spread to your new dragon could easily make this worse. Separate immediately.
:!: Secondly, how big is this enclosure? How old are your dragons? What is your basking temps? How are you taking this temp? What is their diet like?
Personally, I would have not purchased another dragon while the one you currently had was having troubles. I understand you did not really realize this until afterward, but if there's an issue with your husbandry the same thing is likely to happen to your new dragon as well.
Please run us through your setup, as asked above.