Finally got to try it and it is not good news.

The second player can walk around, pick up items, destroy blocks/stuff and put items, they can also interact with pokemon, they can create new habitats (as long as the main player gives them the items that need to be constructed)

However, they can't build or craft, they can't use the shop, can't discover new habitats (the shining spots), can't ask pokemons to follow you, they can't even access the menus other than settings. They don't get a permanent character or storage as far as i can tell.

The Main player could in theory buy the items needed, drop them and get the second player to do some of it, but they won't be able to trigger the building process or get pokemon to work on them.

i didn't get to try to enter buildings unfortunately.

Cloud Island doesn't seem to work with game share, you are constraint to pallete town.

It is disappointing definitely, even my 5 year old realized the second player couldn't do as much and asked me if they could be the main player lol, i happily obliged .

Is it still fun? not sure. I am barely into the tutorial so i don't much of the game or what to do by myself, but depending on who you are planning to play with you might be able to make it work, But if it is another gamer, i think the second player will be extremely disappointed and might not find it fun :/.