Multistreaming Starter Pack
Given the explosion of multistreaming, as someone who's been doing it for years, here's your starter pack:
Stream to https://beamstream.gg as a platform, which has built in restreaming done for you. If you prefer, https://casterlabs.co does restreaming but is not a viewing platform. This lets you have a single output and drastically reduces your required outgoing bandwidth and demand while going to your various chosen platforms.
For chat use https://socialstream.ninja , which not only lets you combine chat across a ton of platforms but also has a built in relay feature, among a metric ton of other functions like polls and stuff. This allows you to have a unified chat that can communicate across platforms, your #YouTube chat can talk to #twitch, #Kick, and so on. And it does have options to let you section off chat, looking at twitch, as you see fit.