I build a lot of boxes for the broadcast industry and there are lots of m.2 expansion cards (they are x2 in length) for all sorts of things, really.
Can't really say without specific examples. (also x2 is not a length, m.2 ranges from 30 to 100mm in length though only a small number of dimensions are generally used, according to the spec sheet, the E keyed slot is meant for 30mm modules)
For full reference the keys provide the following buses
E: PCIe x2, USB 2.0, I2C, SDIO, UART, PCM (I2S)
M: PCIe x4, SATA (though the specifications for this board says no SATA is available), SMBus
Given your mention of broadcast, I suspect you would be most interested in video (DP is provided on A keyed slots, which are uncommon), audio (Provided on B and E), and UART (Provided on E)
While M.2 is designed with a wide range of size and interface options in order to enable system integrators to easily drop in custom modules and upgrade the motherboard and modules separately, the limited number of combinations widely available, means specially designed solutions will only fit where they were designed to. For these purposes this means custom boards will accept whatever the manufacturer wants, but standard boards will pretty much only accept 30mm long E modules and 60 or 80MM long B or M keyed modules
Can both slots be utilized at the same time?
I don't see why not. There's only a few standard connectors, the tallest of which would allow another M.2 device below it which appears to be how this is set up. Also it would be a terrible sell to have a slot for WiFi and then have it block the SSD.
Unfortunately the ultra m.2 looks like it is above the m.2 which makes m.2 cards with pins a problem. I know it depends on the CPU too bit I'm flexible.
That's a fair point, if the lower module exceeds the keepout zone, you definitely won't be able to use the upper slot.
If you're concerned with overlap, mITX, and thin mITX don't tend to stack M.2 slots (though it does happen on some boards)
In my mind the perfect solution would be to mount another M.2 on the underside of the board. It would add a second location to use the Bplus and similar adapters as well as enabling raid configs etc. Perhaps the H170 variant of the board would have the few extra PCIe lanes to pull this off?
Furthermore if a company like AsRock did this and then offered their own M.2 to PCIe adapter, they could place the x4 slot in such a position to ensure that PCIe cards lined up with the STX I/O.
I keep wishing mSTX were patterned after Zotac's mini PCs. CPU on the top, and maybe a MXM slot allowing most of the top to be used for cooling in a standardized manner if so desired (though still allow for some cases to use the standard Intel CPU cooler), and the bottom of the board has SO-DIMM and M.2 slots so you can easily swap them out from the bottom of the chassis without mucking with the cooling solution, and could maybe even fit a 2.5" drive to the board.
