I’m trying to unwrap a character model, and I’d like to be able to modify the uvs after the initial unwrap without losing symmetry. There are tools to manipulate and improve the uvs in the UV editor, but they’re unusable if they’ll destroy the symmetry. Is there any way to do this, or are we just stuck with whatever we get when we unwrap?
Usually when I think about maintaining symmetry in a UV it means keeping two different UV islands overlapped so they will share the same texture. This way something like right and left leg take up less texture space and still look pretty good. You seem to want to have the UVs mirrored within the UV editing. I don’t really get the reason for this if they are not sharing the same UV space they can have completely different textures with them arranged in any way. There symmetry on that UV space would not matter at all unless they overlap somewhere. Usually to save on texture size we would want them packed as tight as possible without overlapping not separate and mirrored on the fictional space. I’m lost as to why anyone would want them mirrored on a UV space, so I’ll just list some tools to help all the UV needs you might want.
Here are some great uv plugins to help you with all your uv needs in blender.
UV Tookit
UVPackmaster
Zen UV
If you do want to get the 2 legs to share the same UV space I think UVPackmaster does this. If you want the legs to maintain the same UV distortion after moving the points you could overlap the UVs, select both UV islands, change their UVs together however you like, then separate them to have different textures on each while maintaining the same UV distortion.
If your model is symetrical you can just use the Symetrize functionality, it mirrors UVs too. If you already has desymetrized you model…
I’m waiting 
I tend to run into this mess myself. Clothing panels being symmetric, run cloth sim, posing, folding, fixing through sculpting and so on. Now I go “oh crap, UVs”. Lately some of the builtin UV addons just crash the daily builds blender instantly.
I recognize the value of that approach, but I want the option of asymmetrical textures on a symmetrical mesh, so this is a time I don’t want the uvs overlapped over the same space. And when I fix one area of the uvs, I don’t want to have to start over on the opposite side (the leg unwrapped fine more or less, I was just using it to experiment. The feet will require more work to even out, as no matter where I put the seams, the uvs don’t unwrap perfectly). I don’t need the final UV layout to have the left and right sides perfectly aligned like this, they can be rotated or offset for efficient packing, but I do need them to be the same size, shape, and density as each other.
I’ve tried overlaying the left and right sides, but moving the vertices together is a problem. It looks like UV Toolkit might have a useful utility for this though.
I just found this today. It dose exactly what you wanted to do. I personally would never use this, but here it is.
Symmetrize UV Utility
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That looks perfect, thanks!
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Almost worked. This method works great for limbs, but not so well for the face and torso since you lose half of it (gets folded over itself I guess).
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