Howdy, Alexander and Welcome to BYC. It is a general rule not to mix different species of pheasants in a mixed aviary. The possiblity of the different males fighting is great and they will fight until one of them is dead. Even hens have been know to fight.
As a rule you usually want birds that would occupy different areas of the aviary, ie; ground dewellers and perching birds.
Although it has been done, I wouldn't recommend doing it. You can keep Ruff pheasants in a mixed aviary fairly well but only if it's an all male bird aviary and a very large enclosure with plenty of perches, hiding places, shrubs, small trees, etc....in other words, lots of "pheasant furniture". Helps break line of sight from another aggressive bird. Also helps alleviate boredom. Another thing to keep in mind if you choose to do it, is to put both groups in the same enclosure at the same time. Preferable, one that is new to both groups. I do this and at night...that way when it gets light their all on equal ground, so to speak. Trying to introduce a new bird(s) to an enclosure with an established group of birds will almost always result in a blood bath.
These have been my experince with a mixed aviary. Im sure others will chime in and tell you their experinces. HTH