There are 3 potential bottlenecks for this:
1- the speed of the harddisk
2- the speed of the interface (USB, SATA)
3- the speed of the OS and the application used for copying
1- harddisks today are capable of ~ 100 MBps, notebooks HDDs as used in external 2.5" cases maybe a bit slower (60-80 MBps). You can attain this speed only when copying sequentially large files, copying many small files will need a lot of mechanical movements of the lever arm in the HDD which will reduce the transfer speed substantially.
2- if your choice is between USB3 (5000 Mbps) vs. USB2 (500 Mbps), always choose USB3. If you need 2 such ports, use a hub. The maximum transfer rate of a USB2 HDD is around 30 MBps, and on USB3 still x10.
3- if you use Windows explorer this can be very, very slow, mainly if the number of files to copy is a couple of 10.000s. Use a specialized application like TeraCopy which uses (it's own) buffering to maximize throughput. An added benefit is that it automatically checks the file integrity of the copy by calculating a checksum.
So, in short: connect both HDDs to the USB3 hub, connect the hub to the USB3 port and use a copy utility.