You see this suggestion coming back every once in a while but I always think it's technically impossible due to streaming/synchronization issues. The game is already asking so much from a computer during a single player game, let alone that it would also have to do it on two computers simultaneously.
Back in the day RCT2 could also not be played multiplayer, decades down the line we got OpenRCT2 a multiplayer mod which allows it and it still doesnt work perfectly. But RCT2 is a much more simple game and isn't as detailed and hardware-demanding as Planet Coaster1/2.
I remember Cities Skylines 1 had a multiplayer mod on the workshop, but I dont think I ever see it work... Maybe it was just a hoax or something. Because people suggested it for Cities Skylines 1 aswell and obviously that's also a very demanding game. I just don't think it could be done easily, maybe you would need a server and 2 clients, because I dont think it would work peer2peer where 1 guy hosts and other guy joins or something. It's just too difficult to make or would be worth it because there would be too many network/streaming/synchronization issues.
EDIT
Source: https://www.youtube.com/watch?v=aYFzwULLrb0
Oh I just checked and it did work according to steam and youtube comments, but many synchronization issues such as one person has a building on fire but the other does not have it. It would be a networking nightmare. But Cities Skylines had mod support and Planet Coaster 1 has none, so I doubt we would ever see this for Planet Coaster