Apple - How can I connect multiple wireless headphones to the same iPhone at the same time?

There is another "workaround" that DOESN'T involve Bluetooth. I often train multiple students students for 5K's, 10K's, etc. Since I train multiple students at a time and often use 1-hr long DJ mixes to set paces for everyone, I connect my iPhone to an FM broadcast device (usually limited from a 30-100 feet range). I additionally, I encourage trainees to use wireless devices such as nano shuffles, etc. that can pick up radio FM frequencies. There's no pesky Bluetooth "pairing" compatibility issues and anybody can join in by simply tuning in to my FM frequency; even those with nothing more than a simple portable radio/MP3 player. Although these devices were originally intended for tuning into your car radio, I've found they work just as well as for other mobile devices that can receive radio FM frequencies. They're not that expensive, either.

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That would be difficult for the conditions described.

Looking for most portable solution,I think this BT splitter might do the job.

http://www.solveyourtech.com/can-two-bluetooth-devices-connected-iphone/

While it is true that there is no built-in supported method for an iPhone to connect to multiple sets of bluetooth speakers, it is possible to do so.

You would need to get an additional device, a Bluetooth splitter. They generally plug in to the headphone jack, and will connect to at least 2 devices. Likely more finicky than using just an iPhone, but possible. Here is one example.