The easiest way is to plug them both into the same computer and copy from one drive to another. If they're USB 3.0 drives and the computer has USB 3.0 ports, that's also pretty much as fast as you'll be able to do it.
But if the computer or drives are only USB 2.0 then it will take longer. If they're large files so that the drives don't have to do a lot of seeking, then you can expect it to take around 70 minutes or so (at a transfer rate of 35MByte/sec).
You can make the transfer go faster if you remove the drives from the enclosures and connect them directly to the SATA ports in your computer, but it will probably end up taking just as long when you include the time for disassembling, connecting up the drives, and then putting it all back again when you're done. And if you have 3TB drives you'll probably run into problems since the USB chipset in the external drives will likely be doing some sort of block remapping that will prevent the file system from being recognized properly when you connect the drives directly.