Getting twin speedlites in a softbox may be difficult where the lights are positioned at the back, pointing in. A softbox that has the lights on a light stand '
within' the softbox is a different matter and often two or four lights can be accommodated this way.
There are several suitable softboxes that would be suitable .... I have one on my Christmas list for Santa to deliver......
http://www.youtube.com/watch?v=jxQgGIj3nqM
I also have a twin light bracket on order with him. It may or may not suit you