It wouldn't require all the animations to be redone, at least not by hand. You could write a script or tool that updates the file to have a second shield image directly above the other on a second layer.
If you don't want to modify then at all, you could display the image by looping through each player's animation frame, finding the shield sprite, and displaying the second shield image above it.
Alternatively, you could loop through each gani, replacing the SHIELD token with a gani. That gani could reference two ATTRs which would allow you to display two shield images.
There's also dozens of other ways.