Synchronising two player serial linked games

[tricky]

I thought of this in the 80s, but have only just remembered.
If you are writing a game that needs fast responses, to make it fair, you should synchronise the two machines so that one player doesn't get up to 20ms more time to respond.
You can do this by messaging back and forth and adjusting the total lines in a frame untill both machines are synchronised - simples
I've never coded this and have no game planned to use it, but thought I would put it out there, or here, before I forget for another 40 years!

[tricky]

If you wanted a simple way to adjust it slowly, you could set vertical offset to 1 to slow down by 64us per frame, this would shift the image down by a scan line though.
Adding a short row after the displayed part of the screen would be neater but require much more accurate timing.