How to setup a two block rockers using a headphone splitter and cables.
There are links to the stuff on amazon:
Block Rocker- big battery powered speaker with inputs
Belkin Y Adapter; Mini Stereo Plug, Headphone Splitter
This was a blog post I wrote responding to a question on amazon.
You could also do this with an RCA jack (like from DVD player), by adding an RCA to headphone adapter.