Connect iPhone to multiple output devices to play music

I don't know about how a Bluetooth device would work in this context, but you can achieve what you want using WiFi and cabled connections.

1) Use an Apple Airport Express with WiFi. Streaming music and audio to a stereo system is what it is designed for.

Go to an Apple Store or other authorized Apple dealer and explain what you want to do and they can show you how it works.

2) Alternately, wire the headphone output from the iPhone into a semi-professional mixer board which was then connected to more than one amplifier and speaker device. This is equipment used by musicians for recording, and it would require some expertise to learn to use.

Here is a link to a list of small, inexpensive mixers on sale at Guitar Center.

Go to a local music store and tell them what you want to do and they can explain how to do it.