Audio Sharing allows for any device that's an iPhone 8 or newer to transmit audio to two headphone sources. The requirements are that both headphones are Airpods or newer beats headphones. This is setup by powering on both sets of headphones. If it's for a set of airpods connect the first set of headphones and then in control center, select your first audio source and there should be a share audio button under your audio source. Select that and it will prompt you to connect the second device. However if both headphones are already connected, simply select both as an audio source in control center and you're done! You'll be able to control volume and any noise control if applicable for both headphones from the same iOS device.