Hello. I am editing olemit's fur coat outfit to have a wider collar and be longer. I wanted the collar to be stiff like shoulder pads, so I separated the coat and collar in order to use 2 different bone references. I also duplicated the back half of the collar and flipped normals so that it looks more solid from the front.

I tried following this tutorial, but running "merge by distance" on the collar seam didn't fix it.
How do I fix the seam and is there anything I should be fixing? I've never made a full outfit before.

The mesh itself doesn't have a seam, it's the texture that does.

It can be partly fixed by selecting each UV island and scaling it down a tiny bit, as there isn't quite enough space stopping transparent pixels bleeding in. It wont be perfectly seamless because the UVs are on different parts of the texture, which don't line up with each other where they meet on the mesh, but it looks better after scaling and probably won't be noticeable ingame. Otherwise, you may be able to improve the edges in Photoshop.

Separating the collar from the coat as a final outfit is not necessary for what you wish to do. It may be easier for you to paint bones with the parts separate, so we don't need to change that. Now, simply put the two geoms together and save a copy. THAT will be your bone reference. At this point, make a copy, and it can also be your working mesh. I did notice a couple of stray triangles. On the UV, one is where the right arm would be, and the other is below the fur parts to the right.

2 things I would do differently with the collar... Working in Blender, while the collar is separate from the coat, delete the backface. You won't need it. Select the edges of the bback part of the collar, near the neckline. Push "E" (extrude) and pule the edge down, and align to the front part of the collar. Merge the vertices where the new section meets the front. (UV merge off). That will cover the area you could see passed the neck.

If you want, you can also do this with the edge that is where the collar rests near the coat. Select that edge, press "E" (extrude), "S" (resize) and move the mouse in a bit. You can resize smaller as much as you need, and move the edge toward the coat a bit.

Select all and merge by distance. Unwrap the UV. Now, select the edges where the front comes together and split THAT seam. Unwrap UV again. In the edit window, select the edges that will split the back of the collar, and split that seam. On UV edit side, set selector to faces, and select half of the UV. Use mirror in whichever direction you need. Move the UV half to overlap the other, align the seam as best you can. On UV side, merge each set of vertices. You now have a seamless UV. Resize it and move to area where right arm was. Don't forget to triangulate.

Make new textures for the new UV layout. This is far more work than what @CardinalSims sugests, and is a matter of how much work YOU want to get into.

OY! That tutorial appears to be for fixing ankle, waist and neck seams. I don't do many am garments, but don't remember having a problem in MTK. Also, Smug Tomato's Blender 2.8 geom tools does a great job of seam fix on export! Works for L2 and L3 too.

I painted a grey border behind those areas and that fixed the seam, thanks!

Hello again. How do I make morphs for the geom? Do I use the "auto tools for geom" or "geom tools" section? In the match morph section in geom tools, the reference morphs section asks for BGEO file; how do I get these? Sorry for all the questions, this is still pretty new to me. Alternatively, is there a way to just turn the combined reference GEOM into WSO?

I always use MTK to make morphs. Then, if necessary, I take them into Blender and adjust them. Auto tools for WSO> Auto-create Morphs. EA af body coat trench trim_fur might be a good donor for your coat.