Desyncs do usually occur when someone has a slight connection derp
It is basically the game going "Hey player 1 is fine but player 2 is almost a month behind them in game time, might wanna fix that"
There's a few things I would recommend
First: Ensure your internet connection can handle both of you actively being online playing games at the same time, plus any other devices in the home that may be connected to the internet.
That is rather simple, as gaming has evolved the requirements of download/upload speed have changed to better handle the more intensive games. And even if a device isn't being used, the fact it is connected to the internet will be a draw. If you think your current speed isn't enough, call your ISP and see what they offer in terms of speed that you can both readily afford. [I'd shoot for something around 50Mb/s at a bare minimum just for the download speed]
Second: Ensure both computers are connected to the internet modem/router via an Ethernet cable. These provide the most stable connections and don't often have the speed limitations that WiFi does. I suggest getting Cat 6 or Cat 7 Ethernet cables, they can handle up to 100Mb/s I think, so they should do you fine.
Outside that if you still experience desync, it might be a hardware problem, where the PC just can't keep up with the game speed. [If that is the case I feel for you, the market is still borked to all hell right now]