Toggle orthographic views opposite sides How to ...
    ... toggle views with the opposite 3d space viewing side ?

e.g.

press key for front view and press again for back view



An half solution is to reuse the same hot key. 


This method has the benefit of quickly reviewing the front and back side. You can keep the hot key pressed to see one side and press shift to see the other side and release shift to go back to the other view.
Another half solution might be to assign double click (possible with keyboard key) to go to the complementary side view.




When these half solutions work fine the operation still seems a bit complicated.
It is script chenge view to the opposite



import bpy
from bpy_extras import view3d_utils
from mathutils import Vector

def getView(self, context, event):
    region = context.region
    rv3d = context.region_data
    #coord = event.mouse_region_x, event.mouse_region_y
    #view_vector = view3d_utils.region_2d_to_vector_3d(region, rv3d, coord)
    return rv3d.view_rotation * Vector((0.0, 0.0, -1.0))

def findView(self, context, event):
    vector = getView(self, context, event)
    if vector == Vector((0.0, 1.0, 0.0)):
        bpy.ops.view3d.viewnumpad(type='BACK', align_active=False)
    elif vector == Vector((0.0, -1.0, 0.0)):
        bpy.ops.view3d.viewnumpad(type='FRONT', align_active=False)
    elif vector == Vector((1.0, 0.0, 0.0)):
        bpy.ops.view3d.viewnumpad(type='RIGHT', align_active=False)
    elif vector == Vector((-1.0, 0.0, 0.0)):
        bpy.ops.view3d.viewnumpad(type='LEFT', align_active=False)
    elif vector == Vector((0.0, 0.0, 1.0)):
        bpy.ops.view3d.viewnumpad(type='TOP', align_active=False)
    elif vector == Vector((0.0, 0.0, -1.0)):
        bpy.ops.view3d.viewnumpad(type='BOTTOM', align_active=False)


class ModalOperator(bpy.types.Operator):
    """swich view to opposite"""
    bl_idname = "object.modal_operator"
    bl_label = "Simple Modal Operator"

    def invoke(self, context, event):
        findView(self, context, event)
        return {'FINISHED'}



def register():
    bpy.utils.register_class(ModalOperator)


def unregister():
    bpy.utils.unregister_class(ModalOperator)


if __name__ == "__main__":
    register()