USB-c multiple ports?

I have a Macbook with an adapter that will allow me to plug in my macbook, an HDMI cable and a USB all at the same time. However, I need to plug in 2 USBs and I have another adapter where I can do that, BUT, only it seems that my Mac is only recognising ONE of the items plugged it. The items are 2 external storage devices....one back up, and one external storage. It's backing up right now, but I can't get the external storage device to come up in finder. Both are brand new.

Does the USB-C only allow for one item to be used at a time?