You don't need two PC's to stream to be a successful streamer, but it does have 2 big advantages:
Archie 'Parallax' Shrader is a full-time Twitch streamer. He is currently focusing on Fortnite.
Parallax uses the Razer BlackWidow TE Chroma V2 gaming keyboard.
For gaming, Parallax uses a Intel Core i7-7820X CPU with a EVGA GeForce GTX 1080 Ti SC2 GPU. He uses a ASUS TUF X299 Mark 2 motherboard. Parallax uses G.SKILL Trident Z RGB Series 32GB memory.
For streaming, Parallax uses a Intel Core i7-7700K CPU with a ASUS ROG Strix GeForce GTX 1080 GPU. He uses a ASUS Z170-A motherboard. Parallax uses Ballistix Elite 16GB memory.