You don't need two PC's to stream to be a successful streamer, but it does have 2 big advantages:
James Chris 'Replays' Crowder is a full-time Twitch streamer for FaZe Clan. He is currently focusing on Fortnite.
For gaming, Replays uses a Intel Core i7-8700k CPU with a ASUS ROG Strix GeForce GTX 1080 Ti GPU. He uses a MSI MPG Z390 Gaming PRO Carbon motherboard. Replays uses CORSAIR Vengeance Lpx 16GB memory.
For streaming, Replays uses a Intel Core i7-6800K CPU with a NVIDIA GeForce GTX 1070 GPU. He uses a ASUS ROG Strix X99 Gaming motherboard. Replays uses CORSAIR Vengeance Lpx 16GB memory.