Bad news. Reading from two SD cards in the CalDigit SOHO is slower than reading from a single SD card. Very disappointing. I guess the SD Card Reader USB device is not smart enough to handle two devices at the same time efficiently. Maybe this is something UASP would help with.If you have another SD cards maybe you can write to microSD V90 and another SD card ~10GB data. After that copy from two cards to Mac. Activity Monitor in "Disk" you will see speed of disk whrite if you dont have another monotiring app who show each disk/card speed.
If speed will be more what you get from single microSD cards read speed (from your test it was 276-290MB/s) it's will be WIN!
I received a SanDisk Extreme PRO SDHC UHS-II Card 32 GB (300/260 MB/s read/write). It comes formatted as FAT32 (274/64 MB/s read/write) with MBR. I don't know why macOS has slow writes in these benchmarks but we care more about the read speeds. I reformatted the SD card as HFS+ (296/120) with GPT.
ATTO Disk Benchmark.app gets similar numbers as AmorphousDiskMark.app for single SD card (write speed might be better in ATTO because we're just writing zeros). When I switch to dual SD Card, the numbers go down to 115/53 MB/s read/write.
The Finder test is not as bad. It's 274 MB/s for a single SD card and 245 MB/s for two SD cards for reading. I think it was around 100 MB/s for writing. The files used are 76 screenshots of various sizes between 4 KB and 269 MB duplicated 15 times. 11.24 GB total.
So I guess now the question is, does any dual slot SD card reader behave better?
Maybe a dual slot SD card reader that is a single USB device exists that has ideal behavior but the CalDigit SOHO is not one of them.
An SD card reader that consists of two separate SD card reader USB devices connected to a USB hub should be ideal but do any such SD card readers exist? Maybe the Prograde is one of those.
I've attached some screenshots. I edited the Finder screenshots by removing the background so the .png files would compress better.