The use of double counters in photon counting to avoid dead-time

This is less of an issue with regards to the measurements and more of a "what's the idea behind it?" . In Hamamatsu's Photon Counting Unit, the C9945-13 series, they reference double counters to allow measurements with no dead-time. This wasn't so clear as I figured with the photon counting head and say a PCI6602 counter/timer card, Lab View and a setup similar to the one in this post attached:

https://forums.ni.com/t5/Counter-Timer/Photon-Counting-Time-Stamping-and-basic-help-with-how-DAQmx/td-p/224447 

there isn't a clear need for 2 counters unless 1 counter is storing the number of events while the other facilitates the storage of the time tag. So my question boils down to what the use of the double counters is for with regards to dead-time in measurements?