To paint both sides of a symmetrical model simultaneously, it must be symmetrical or be posed in a symmetrical manner (in the case of a character model).

You can paint on both sides of a model as follows:

• If the model is positioned exactly at the X, Y, Z origin (0,0,0) in the 3D View, select the Paint Brush, then set the axis you want paint mirrored across in the Mirror drop-down list in the Properties window.
• If the model is not positioned exactly at the origin in the 3D View, select the Paint Brush, then select the local X, Y, or Z axis you want paint mirrored across.
• You can also apply paint in a symmetrical fashion when using stencils.