Only way I've ever managed to retemper was by following instructions in a book. Heat gently and add a little vegtable oil slowly, whilst stirring and it often recombines. It's not suitable for much though, I feel.

Those double wall steamers are nice, but a right pain to clean. You can get steamer/bain marie inserts for common pan sizes. Stainless steel, one piece, cheap, easy to clean, unbreakable and as they fit properly they work really well.