Return current path follows least resistive path in DC signals; least inductive path in AC signals as you seen in Figure 1.
In Figure 1, there is no GND pour on the top layer, and only a poured GND on the bottom layer.
In Figure 2, there are GND pours on both the top and bottom layer.
What happens if I pour GND on the top layer and connect each GND polygon with vias as you seen in Figure 2? I mean, does the current path goes to its source on the top layer and bottom layer at the same time? If yes, how much of return current goes to top layer and bottom layer?