Stop marzipan drying out during storage (eg. on Battenberg)
Chill it in the fridge for an hour or two (aids slicing) before you open it, then slice it all immediately.
Wrap each slice separately &amp; freeze.
I'm not sure I'd want to eat 2-week-old sponge cake whether it was wrapped in marzipan or not, kept anywhere except a freezer. This type of product is only shelf-stable until it's opened, after that its life-span is short.