I've no experience on Duskys so I'll save that for others. I do know that suns tend to get rather jealous and possessive. That being said, I wouldn't necessarily rule out getting one. It's more a matter of preparation and priorities. If you believe you can tend to the new suns needs without sacrificing your duskies needs, that's the thing. Other than that - anytime you introduce a new fid - there are no guarantees. And there is also only so much you can do. Some birds won't take to another no matter what we do while others take right to one without any effort from us at all. Bear in mind though, that the new bird will likely alter the bond you currently have with your dusky. How it will alter depends on your bird.
Whenever you introduce a new fid, you want to go with the quarantine first followed by a vet checkup. Doing that does require a separate cage. Once that is done, I think the best thing to do is have your sun kept in that separate cage (provided it is of appropriate size for the species) and then just keep them both in the same room and be sure to supervise them when they are out together. I'd introduce them gradually. If you just plop the two together in the same cage from the start you are really taking a major risk that is more 50/50 at best. Not worth the risk, IMO.
If once you've had them out of the cage supervised they are getting along for a good period of time, you could go ahead and try co-habitation. But in my experience, every bird prefers their own 'space'. I would think that would be true especially for mixed species.