Hello, I saw the post regarding this mod and because I played around with it I want to share my findings.
I have the latest version of the mod, I don't know if whatever I say will be true for other older versions.
I'm going to refer to the two players as "H" for host and "P" for the other player.
First off DONT sell anything at the sellbox, if you do at the end of the day when both players go to sleep "P" will be stuck at a black screen, the ingame character will be in the bed. If you sell anywhere else it will work fine.
Second rarely when transitioning to another area "P" will get stuck at the loading screen. The solution is you have to tap where either your quest button would be or the button with the three lines, this will open the corresponding window and once you close it you will be out of the loading screen.
Third If "P" joins for example in day 2 the game will consider it P's first day. So if the community center opens for "H" "P" should NOT try to get in, the game for "P" will either straight up close or take you to the main menu.
Fourth "P" from what Ive seen cannot get mails, I don't know about gifts but mails don't get sent to their house. But if for example you go to the beach at the 3rd day for "P" it will trigger the fisherman cutscene.
Fifth This has ever happened once to me and I can't replicate it, "H" is not able to pick up any item off the floor.
This might have happened because both players where collecting items at the same time but I'm not sure.
Sixth Player's location isn't updating. I don't know what causes this or how to fix it, but pretty much the mod stops updating either players location, that causes, from my experience, "P"s time to completely freeze and so neither of the players can sleep. The players can move around but from the others perspective they are standing still, and can't interact with chests or anything else. Transitioning to another area may cause a crash or a freeze.
If I find anything else that I believe is important I will update this post. I hope that helped!