Trying to plug 2 external hard drives into my early 2016 macbook. one for photos and one for time machine.

Hi. Just got my early 2016 MacBook. I use the photos library and store all my photos and videos on an External Western Digital Passport HD. I purchased a USB-C adapter that allows me to power the machine as well as plug in 3 USB devices. I'm able to sync my iPhone photos directly to the external HD, however when I try to back up my WD Passport and MacBook to Time Machine (WD My Book 4TB drive) ie. using 2 USB ports, the back up will happen for a couple of minutes but then it says my Passport HD has not been properly ejected. Any thoughts? Many thanks