How to link nodes by translation

Hi,

I have a lot of nodes in one language, and their counterparts in another language.

When you click at their 'translate' tabs, they show up in the correct language row, but their counterpart is missing.

Is there a way to add a node in language B to an existing node in language A? Is there an interface for that, or can it at least be done by editing MySQL records (how)?

Thanks!