You will need to get 2 compatible powerups to their max level.
Then, the next time you level up you will be able to fuse them.
Somebody posted the full list of valid combinations on twitter using the game hashtag #yeetirys
Have fun!
Viewing post in Delivering Hope (hololive fangame) comments
For others' (and my) convenience, this is the tweet: https://twitter.com/isaac_wolfu/status/1628357132167913472