The above cable is exactly what you need plus...
2 lots of.......1male into 2xfemale rca connector
2 cables of 3.5mm headphone to male rca’s
This set up in photo is..
2 x Bose soundDocks (both have 2 speakers..IE:stereo)
This cable system ($30) bought from electronics store..Jaycar in Australia. Is easy to get done.
Both sets of speakers are now ...