I don't have any disabilities myself, but a character I'm working with does (a man with erbs palsy), and he's the antagonist. I didn't realize that giving villains disabilities was a trope, but he's always had this disability since he came in my head, and I think it would be wrong to remove it. The main character also has a disability (PTSD), and I was wondering if this... cancels out? I want to make sure I'm not furthering the weird prementioned trope.
There's a lot to consider here.
For one thing, it can't exactly "cancel out" because it doesn't really work like that. Disabilities aren't one size fits all. They're different and when it comes to representation, each one comes with its own history of stereotyping, ableist tropes, and social context that needs to be considered.
With this in mind, there are some MAJOR differences between a character with PTSD and a character that has a physical, visible disability -- especially something like Erb's palsy that causes visible differences in their body.
Characters with visible disabilities (and especially those whose disabilities cause visible differences in their body) are almost always portrayed as the villains in their stories. Think of characters like Freddy Krueger (Nightmare on Elm Street), Dr. Poison (Wonder Woman), and most Bond villains.
I don't think I've ever seen a character like this in any role but the villain.
Meanwhile, there are plenty of characters with PTSD who have other roles. Tony Stark (Iron Man) is a hero, Judd Ryder (911 Lonestar) is one of the main protagonists, several characters from the Hunger Games. While I'm sure that are villains with PTSD as well, they're far fewer.
There's also the issue of how villains with PTSD are portrayed versus those with visible disabilities. Many 'villains' with PTSD (Bucky Barnes/Winter Soldier, Jason Todd/Red Hood, etc.) are portrayed as characters that have been through a lot and need healing/love. They usually get a redemption arc.
On the other side of this, visibly disabled villains don't get that. They're portrayed as simply needing to be stopped.
I have... a lot more to say on this subject but the point here is that no, it doesn't "cancel out". If your disabled character is problematic or ableist in some way (which I can't say based on what info is included here), that isn't something that can just be magically fixed by throwing in a character with PTSD.
There are a few things you can do to make some improvements to your concept, though.
First, I'd suggest checking out some of our posts on writing disabled villains. You can find most of them in the pinned post or by looking through the #villains with disabilities tag.
They have a lot of good advice that you might find beneficial.
If you'd like to send another ask with more specific information on your character after you've done that, then we can help with workshopping them.