According to the information currently out, "hybrid dual-SIM". Better description for such would be "hybrid not-really-dual-SIM". If you can only make use of either a SD-card or a second SIM, you're cutting off your right arm to become ambidextrous.
You can readily find information on how to make use of both features at the same time, remove the extra plastic on a SIM and then glue it to the bottom of the SD-card. The fact that this is so well circulated seems a strong recommendation that this is not a desirable feature. Make the base of the carrier 0.76mm thicker and both features work together without modification (minor modification required for the contacts, but I imagine that would be simple).
So a mere extra 83mm^3 for such a large improvement doesn't seem like a big deal. This isn't the original iPod where Steve Jobs demonstrated there was wasted space by dropping it in a bucket of water and observing that bubbles came out.
The other problem with the hybrid slot is there is a potential need to completely power the phone off to do anything. It depends upon the device, but both SIMs and SD-cards can be hot-swapped. Many phones are deliberately designed to prevent this, but the potential exists. Potentially one could have a full load of Debian on a SD-card, but when you want to hot-swap you're stuck.
The ideal phone I would create if I ruled the world would have two SD slots. One would be the OS card which the phone booted off of. There would be no potential to hot-swap this, but would allow upgrading the card Android is on to something larger than 128GB. Then have a dumb card for music collection or games/whatever.
