I have an iPhone 6. How can I use two Bluetooth audio devices and the 3.5mm audio jack together?
As far as multiple pairs of Bluetooth headphones simultaneously, no, not without additional hardware. While you can obviously pair your iPhone with multiple Bluetooth devices, it can only output audio to one Bluetooth device at a time due to limitations in the A2DP audio profile that is used to transmit audio to the Bluetooth device.

You also cannot output audio to both wired headphones and a Bluetooth audio device at the same time. They are separate options on the audio output menu in Control Center ("iPhone" is the output setting for wired headphones). This was determined through my testing.