Depends on the usb chip and firmware that is in the multi-bay docking station what actual transfer rate you will get from drive to drive in the same dock. This is next to impossible to determine unless you have the docking station to test.
Fortunately someone did this for one model of multi dock. Drive to drive in the same dock are in Bold
My experience shows that I get about a maximum of a gig or better per minute on large file transfers from one usb hard drive to another usb hard drive on separate hubs, so it looks like for this particular dock it is about twice as slow.
All files used in my test were .mp4 movie files.
Acer computer to Hitachi Drive
253MB copied in 19s
699MB copied in 34s
Acer computer to Western Digital Drive
253MB copied in 15s
699MB copied in 35s
Acer computer to Hitachi Drive using multiple files
1.35GB copied in 64s
Acer computer to Western Digital Drive using multiple files
1.35GB copied in 63s
Hitachi to Western Digital using multiple files
1.35GB copied in 119s
Western Digital to Hitach using multiple files
1.35GB copied in 122s
Hitachi Drive to Acer Computer
1.35GB copied in 59s
Western Digital Drive to Acer Computer
1.35GB copied in 59s