How to answer smart phone from multiple bluetooth headsets
You cannot have multiple headsets ringing at the same time by pairing multiple Bluetooth headsets to a single iPhone.

Various solutions exists for having multiple headphones connected to the iPhone at the same time. For example in earlier days you had headphone-jack splitters, then Bluetooth splitters - and now Apple has their own built-in support for multiple listeners.

However that is all it is - multiple listeners. Even with one of the external Bluetooth splitters available for the iPhone, you can only listen to audio - it doesn't work with the microphone.

In order to achieve the effect you want, you'll need to have two cellphones, and have your provider set it up so that both ring when a call comes in. This can be done via traditional cell phone technology, or it can be a Voice over IP solution - in that case you could also use an iPod Touch, iPad or other device.