It looks like it is either SD Card or Sim Card and not both at the same time with the way the slot works. (People who have it can confirm whether or not it's true.) But I have a theory of how to get both at the same time.
With the above photo, you can see that I took the sim card out of they plastic using a hair dryer, some scissors to cut the edges off, & a knife to separate it from the plastic.
We can then take the card & attach it to the back of the SD card. Theoretically it should be able to read all 3 cards at the same time. The pins are clearly visible for the SD Card and both sim cards when in the slot. I'm in the US so I won't be able to test my theory out until it launches here, if someone wants to try it and report back that would be awesome! I'll end up getting this phone regardless, but if this works it'd just be icing on the cake!
Edit: Thanks to 'jkuczera' we have a couple videos for you to see it.
We have confirmation that the mod works for the Axon 7 guys! 2 Sim cards & an SD card!
Part one of two [youtube]AgLfuGtieic[/youtube]
Part two of two [youtube]Hoh5SyOLmBA[/youtube]