My computer cant detect 2 external hard disks

I have 2 western digital 1 terra byte hard drives and they both work fine when I connect them independently to my computer but now when I wanted to transfer data from one onto another I plugged them into my USB ports but only one will show up in my computer.
I tried reconnecting them to different ports than those I used earlier but still the problem persists. It won’t be an issue of power since I am connecting both of them to my laptop. Since I have tried that also with my laptop connected to the charger, I would be glad for any help?
