Hi @HardWater
I believe that you can force a Photon into DFU mode by opening the serial port at 14400 baud on the host side. There are a couple of threads about how to modify the Particle CLI (if you want to) in order to force that.
The limitation of DFU only targeting one device at time in DFU mode is a limitation of the DFU-Util program, I believe, and is not related to Particle.