Is it possible to connect two iPhones to the same external GPS device?
Yes, this depends on the Bluetooth device.
Also note that there's a difference in pairing multiple phones with the same device, and actually be actively using the device from multiple phones at the same time.
I do not think it is common for Bluetooth devices to allow the latter in this use case, however it is ofcourse possible to implement for the device maker (at the extreme including two separate Bluetooth modules).