What I always do, in situations like this, I make a simplified for the behind model part, with 1px black texture with no map. This is what I do when some detail need a "double sided" mesh.

But in your case, you can duplicate your "main mesh" and then decimate the copy. After this, link the edges and use the same map. This is work for the LOD0.

The LOD1 don't need the double sided, then you know what do in this case. =)

That's a good idea, thanks for replying

to answer your question most directly, would YOU accept your one sided cape? :)

i agree with akaize's solution regarding a lower-resolution backside, especially if you have no better option and few tris to spare. i'd also keep in mind rigging/weight mapping and the positions of valve's preexisting bones when creating the lower resolution model (assuming drow's model has bones for her cape) so as to avoid any possible artifacts or glitching from one side of the cape clipping with the other side during animations. definitely check it out in-game to make sure you dont have any major problems with this.
(note: a lot of items seem to have been allowed in game with -minor- issues like this, but dont use that as an example :)

as far as uv's and textures go, i'd try to allow at least a small space for the back-facing section if you arent able to mirror the uvs with the main side. despite what akaize recommends, i suggest NEVER using a flat color/texutre for any area of your mesh, unless you are absolutely sure nobody will ever possibly see it. also remember in dota 2, every color texture space also correlates to normal, mask1 and mask2 texture spaces.

as if i haven't already written enough -- i'd ignore lod0 as far as dota2 items are concerned. the lod0 will never be seen except in the portrait box ingame. if you are working on an item that will be particularly obvious in the hero's portrait, it may be worth considering, but also be aware it'll use the same exact textures as the lod1. for almost every item, a lod0 is essentially useless and probably a waste of time.

-in short, yes, you do need seperate geometry for thin sections like capes that can be seen from both sides. all valve's dota2 hero shaders (as far as i know) do not support or render back-facing polygons :P
(FOR SOME REASON, THEY DO RENDER THE INTERIOR OF TREES; check that one out yourself - lol)

i'm sure this was more information than you were looking for, but i hope some of it helps!

This help is more than enough, thanks !

lumiére, that 1 px texture, is for little detail and mesh, that you don't want to like transparent from behind.
http://www.dota2.com.br/threads/refazendo-set-strength-of-the-futures-gods.20434/
Like the shoulder bottom part, the back side is all black.

I'm working on this set, then don't look to much to the textures.