Depends on what you call “simultaneous”. Short answer: no, the DW3000 (like most other radio technologies) is not able to transmit while receiving or to receive multiple frames at the same time, so it is not able to range with multiple devices at the same time. Most likely the Apple U1 chip also does not support this, but it is not our product so I don’t know for sure.
It is however possible to use e.g a TDoA scheme to range with multiple devices sequentially, and since UWB radio is fast this can be done quickly which is typically good enough to appear as simultaneous ranging. The IEEE802.15.4 spec allows sending broadcast messages so the same frame can be received by multiple devices, and the DW3000 has special feature such as frame filtering to facilitate creating a IEEE802.15.4 RTLS system.
E.g a ranging initiating message could be sent from the DWS3000, picked up by all the iPhones at once, which could then sequentially send a response with their timing information.
I don’t know what RTLS schemes are supported by the Apple Nearby Interaction system, though afaik BLE is also required to perform some out of band communication.
Note that the DWM3000 module is just the DW3000 IC with an antenna, crystal and other components required for RF operation. It does not have it’s own microcontroller or UWB stack. What is possible in terms of ranging with multiple devices almost wholly depends on the UWB stack, i.e the firmware. The DW3000 radio chip can be quickly reconfigured to communicate with different devices in sequence, but the firmware needs to tell it to do so. It’s just a radio module, to make s full fledged Apple Nearby Interaction compatible product a microcontroller, firmware and afaik a BLE radio chip need to be added.