Or an even better way to do it…
https://www.motionscript.com/design-guide/invisible-facing-away.html
Use the expression for the opacity for each side of the postcard:
if (toCompVec([0, 0, 1])[2] > 0 ) value else 0
import each one into the comp, rotate the backside 180 degrees on the y-axis, apply the expression to the opacity on each layer, create null object and parent each layer to it. Using the expression method, you wont have any issues if the postcard is far away from the camera, or if you use depth of field, or if the layers have motion blur applied to them. Good luck.
Jon Geddes
Motion Graphics Designer
http://www.precomposed.com