How to have sorting order/layers used when Z depth is the same?

I've got a shader which is simply based off the standard shader with a few tweaks here and there for use with 2D sprites in a 3D environment. My goal is to allow depth rendering (so Zwrite is on and ZTest Less) however, when it comes to armour and layered body parts I would prefer not to offset the armour for it to render in proper layers.

Currently, the sprites and armour just clip as they are in the same plane, whereas with ZTest off, the layering works but the depth does not (obviously). Ideally I'd like it to use sorting layers to render things when two sprites have the same depth. Is there anyway I can achieve this effect? Hopefully I was coherent enough describing the problem. Thanks!