Some of the Micro-SD slot based low-cost USB devices , actually can use only single slot at a time,
some can only use two at a time, etc.
So they may have 3 or more or multiple different types of slots : SSD, MMC, etc, etc.
But memory-card that is inserted 1st, is used by the device.
These devices have single internal controller chip to handle multiple slots, so they have these restrictions.
So you will have to use multiple (low-cost) USB devices, to use multiple SD cards, etc.
There are some Memory-card-to-USB converter/adapter device, which can handle two different type of cards at a time simultaneously,
but cannot handle same/similar two types simultaneously : Mini-SD or Micro-SD.
I'm sure there are multiple memory card to USB converter adapter/cable device,
that can handle multiple same cards simultaneously, if they have multiple same card slots,
so these are using multiple hardware controllers inside it, for each memory card slots.
imho, using (low-cost) multiple memory-card to USB adapter/cable is better solution.
in that way, each channel remains separate.
And to make it more better, do this:
If these USB devices are connected into PC/mac's different/separate USB ports, which have separate USB controller, then, user can get full USB bandwidth copy read/write speed, for each memory device.
But if both/multiple cards are connected into PC or mac's same USB port, then their max read/write speed will be slower, as they are sharing same USB port's same max speed.
Memory cards also have limitations, some are very fast & some are slow.
So use faster memory card,
and use separate USB ports in your computer.
But, if your memory cards are very slow speed version, then using shared USB port is okay.
