I've never bothered with back up in the freezer, but when DS was 5 weeks old I had a night out.
Before I left I put a grumpy, not hungry DS to the boob, just to get letdown happening, holding the cup under the other one then would take him off & hand pump.. when it started to slow, I put him on the other side & did the same...

Ok, that confused me, but hopefully you get the gist??