Yes, this makes sense. But make sure, the external drives are not connected to the same bus at the end. Sometimes, even if you have 2 USB ports, there are glued to the one inside the computer.
The answer to your question does rather depend upon the machine you are using. If you are using a laptop then Martin is absolutely right. However, if you are using a desktop then it is better to have more internal drives. A Solid State Drive for the operating system, a project drive, a sample drive and a storage drive. The sizes and type of drives depend upon your preferences. If you are using samplers that stream samples on a USB drive might face bandwidth problems if you are using a large number of samplers, unless perhaps you are using USB 3 compliant external drives. Although to be honest you do have to use a lot. For instance BFD3 and a number of Kontakts running lots of different instruments.