Making liner – Normals inside

In 3DCG, face has “normal vector” which means “visible face” or “outer material”.
PMX model also has normal vector on face however MMD allows “両面描画”(both sides drawing) option by PMX editor, you can see “liner” by this option.

No liner – no both sides drawing option for skirt

Liner – by both sides option for skirt material

PMX Editor “両面描画” (both sides drawing) option

In importing pmx model to blender, even though “both sides drawing” option is checked, you can NOT see inside normals(liner).

in edit mode, press [N] and check face icon in [Normals]. blue line is normal vector.

When you export collada model to secondlife, no normals materials get transparent because there are not anything as faces.

Therefore you should make new faces which have inside normals.
By using materials selection, vertex group assign or anything, select faces you want to copy.

Press Shift+[D] to make duplicate and [Enter].
And press [Mesh] – [Normals] – [Recalculate Inside].

Check inside normals.

In terms of “Secondlife Collada Exporting”, sometimes you can see huge number of “land impact” by this approach. For example, when I upload this skirt mesh model with liner, I can see 308.048 even though LOD low and lowest are 0.

Land impact is not simple vertex count. It depends on shapes.
If you see huge number of land impact by making liner, you should separate liner as another object in blender.

When you duplicate inside faces, you should assign to new vertex group or separate vertices by selection. And select all objects and try to export it as collada.

You can see 9.074 land impact in the following snapshot.

Also liner is not transparent in-world.

Good luck.