Many Blackmagic Design capture devices can be used as either an input or output device. One example of this is the DeckLink Studio 2 card, which came preinstalled on Livestream's first generation of Studio hardware products (HD50, HD500).
Add this card as an input by going to Settings > Inputs, then click the '+' button to add it with whichever connection type you are using.
If you add this as an input, but then try to add it as an output by going to the 'Outputs' tab, Studio will notify you that it is already being used in Inputs.
If you want to use any device as an output, be sure to remove it from Inputs by selecting it, and then clicking the '-' button. Then you can go back to the Outputs tab and select it.
When you select it as an output, you will notice that if you try to add it as an input again, Livestream Studio will notify you that it is being used in Outputs.