The halacha is that husband and wife not sleep in the same bed during niddah. This is usually understood as requiring separate bed frames.
A few factors arise in translating this halacha into practice:

The beds should look separate when the couple sleeps.
There should ideally be enough distance between the mattresses that the bedding not touch and that the couple not touch when moving around in their sleep.
Neither spouse should feel the other’s movement from one bed to the other.

 
It is permissible to have beds constructed to join together when the couple are permitted and to roll apart during niddah. Such beds can be linked by a shared headboard. Some halachic authorities also allow for separate mattresses on a wide platform bed with no surrounding frame, treating the platform as a sort of elevated floor, and each mattress as its own bed, separated to ensure meeting the above conditions.
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