Splitting audio between Bluetooth and cable

I have two audio systems at home: a docking station and Soundbook by Bayan Audio. The former allows only for wired connection, the latter supports both the wired and the Bluetooth connections. With my iPhone 5 (iOS 8) I would like to stream music to both devices simultaneously, however currently it does not work.

I've tried the following setting: I connect my iPhone and the docking station via a cable (the docking station does not support the Lightning connector), I get music from the docking station. But when I pair iPhone and Soundbook via Bluetooth, the music stops at the docking station and only plays on Soundbook.

I guess the reason is that iPhone can only stream audio either via cable or via Bluetooth, but not simultaneously. Is that true? In that case, is there a way to fix this issue? Perhaps, it would work differently if I use a dock connection on the docking station, rather than connecting it with iPhone via the cable?