US firm LumoPro have announced an adapter that allows two speedlights to fire into a studio softbox. The LP739 Double Flash Speedring Bracket is “designed to mount two hot shoe flash units into most softboxes on the market… up to 4’x6′”.
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The LP739 has two extending arms which let you mount one or two flash units, with or without radio receivers, into a speedring of a softbox. Each arm can be adjusted with a thumbscrew. The coldshoe mount on the bracket is “specially designed to prevent shorting out the shoe mount connections”. Total weight is under 1kg and the kit comes with a carrying case.
Retail price for the LP739 is US$45 or €50 and it is on sale now. LumoPro products are sold in the USA, Europe and China. Visit their web site for a list of dealers.