I can't answer for Flex, but if you think about it, when you install SmartSDR to your PC, it doesn't know which radio you are going to connect to. It only "find out" when you the run the program to attach to the radio. Some people have multiple radios, like a 6700 (that can talk to 8 slices) and a backup 6500 (that can only talk to 4)
As the 6300 only has two slices, then you can only "talk" to two DAX devices. However with the single install, you can talk to multiple radios and the DAX channels are available in the PC, for what ever you have.
Interesting point. Seems like one can only assign one DAX channel to each slice. However, you can delegate more than one piece of software ware to the same DAX channel. I am using a single DAX channel to stream audio to my iPhone using the Broadwave streamer, and the same channel to my Romac equalizier. I can't see where it would be to my advantage to delegate more than one DAX channel to a slice. However, it wouild be nice to be able to have more control over the remoted audio stream. It seems it can be attached only to the default speaker designee. It would be nice if it could be singularly identified like a DAX channel so it could be associated directly with multiple third party software.