To paint both sides of a symmetrical model simultaneously:

• It must have UV texture coordinates (UVs). For more information see Prepare a model for painting.
• It must be symmetrical or be posed in a symmetrical manner (in the case of a character model).
• It must have a new or existing paint layer to paint on. For more information, see Create a new paint layer.

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 3D Paint brush, and in the Properties window, select either X, Y, or Z (depending on which axis you want your paint strokes to be mirrored across) from the Mirror drop-down menu.
• If the model is not positioned exactly at the X, Y, Z, origin in the 3D View, select the 3D Paint brush, and in the Properties window, in the Mirror option, select either Local X, Local Y, or Local Z (depending on which axis you want your paint strokes to be mirrored across) from the Mirror drop-down menu.
• You can apply paint in a symmetrical fashion when using stencils.