FlockEye makes several iOS devices to take picture at the same time, then it collects those pictures and make an image to be shared.
Open FlockEye on the devices you would like to take picture with. Make sure bluetooth and Wifi is active on those devices. Choose one of devices as master and wait for the connections were established. When you saw red dot touch it to take pictures. But you might want to wait till all the slaves are connected. Now, wait while all the pictures are being received by master and that is it.
You can send the constructed image to photo library or can share it on Facebook or on Twitter. Just touch sharing symbol.
It is able to select any camera on your device and can work on iOS11.