Is it possible to symmetric weight-paint an asymmetric mesh? Is it possible to do symmetrical weight painting without manually splitting my mesh into two pieces? I've already done building my mesh. I don't want to split it into two pieces because:

a) My mesh is not symmetrical to start with

b) It's gonna mess up the rigging on other parts

There are certain parts on the mesh that ARE symmetrical which I want to do symmetrical weight painting. What I mean is when I paint on the left side, Blender draws an imaginary line across middle of mesh and the right side will automatically get painted the exact same.

What I mean:



I want to be able to weight paint left side of that indicated area, then right side will automatically reflect same weight painting. But as you can see even though the upper part is symmetrical, entire object is not symmetrical. No this is not my object I just quickly drew it in MS Paint to show what I mean.
EDIT: According to person asking this question, this is the exact opposite of what they wanted, all i can say is just tick it on instead of off, but they said that doesn't work, i shall keep this up for those who find it though.

This is because X Mirroring is on, to turn it off go to into the Weight Painting mode on your desired object. 

On the left Toolshelf T select the Options(1) tab.

Under Options(2) Uncheck the checkbox labeled as "X Mirror"(3)



Your weight painting should be fine now! 

Tip: open up a new file, without changing anything (other than what i'm saying now), even moving the camera, repeat the steps above and then with the mouse over 3D view, press CTRL+U and then press "Save Startup File"

This makes your blender startup file have what you just did already done, so you don't have to do it again.