I have single object (human model), but I want legs and arms to be in different layers so I can composite it in compositor. I don't want to break it into different object.
You can't move part of an object to a different layer. You can split it into 2 different object by selecting the vertices you want to separate in edit mode and hitting the P key separate by selection.
Now that you have 2 objects, in object mode place them in different layers by selecting the objects you want to move and hitting the M key then a number (not from the keypad).
Use a Number to select from the upper row or ALT + Number for the lower one
Objects can be moved from one layer to the next. Mesh elements(vertices, edges, and faces) cannot, since they are apart of the same object.
The only option I can think of for your situation is to use vertex groups.
You cannot move different parts of the same object to separate layers.
As mentioned by sambler, what you are trying to do is possible using separate materials to generate masks.
Quoting from my answer here:
This can be done . . . using an ID Mask node:
. . .
1. Enable Material index in Render layers > Passes, and set the pass index for the material:
Use Materials > Settings > Pass > index to set the pass index of the material in Cycles:
Or in Blender internal use Materials > Options > Pass index:
Add an ID Mask node and set the Index to
Plug the IndexOB output from the Renderlayer node into the ID Mask node. This will provide a mask for the [material]:
This will give you a black and white mask of that material, which you can use to separate, combine, etc. in the compositor using mix nodes.