Applies to: Scarlett 6i6, 18i8, 18i20 and Clarett 2Pre USB, 4Pre USB and 8Pre USB when using Windows
When using some pieces of software such as Skype, GoToMeeting or streaming software such as OBS and XSplit you'll notice they have a limit to 1 or 2 inputs and outputs.
In order to use more than one hardware input with these programs you'll need to do a physical loopback. This involves connecting a cable from one of the outputs of your interface to one of the inputs.
For an in depth guide on this please see these articles: https://support.focusrite.com/hc/en-gb/articles/115000794545-How-to-create-a-Skype-Mix-for-Scarlett-2nd-Gen-or-Clarett-Red