We recently built and commissioned a fridge and freezer on our boat, with just 1 cold plate, in the freezer. Ours is about 80 litres freezer and 160 litres fridge.
We put a 25mm insulation
panel in between the fridge and freezer (glassed on both sides, coved into place. We put 2 holes between, and installed plugs (actually large dinghy
bungs). We put a small computer type fan in the freezer that runs whent he compressor
So far this seems to work well. If we find we aren't able to keep the fridge cool enough, we will put another hole and plug