blastar (./5) :
The hardest part was the map/graphics data... I use many tiles multiple times with different palettes. To make this possible, I had to create my own tool.
In the end, I invested more time in creating some tools (including a special Z80 driver) than in the NeoGeo code itself.
The trickiest part was implementing the split screen and displaying the individual cars on all screens depending on their position!

Very interesting and yes I can imagine that working out the car positions would be hard! It sounds like your next game game could benefit from your new tools there

blastar (./9) :
The hardware does not support split screen, to implement this feature a few (simple) tricks were necessary, I don't want to explain everything but clipping right/left works completely differently than clipping top/bottom.

On other consoles, they often require large black borders between player screens to facilitate changes to hardware registers and VRAM etc. I heard this is the case on Mario Kart on the SNES. Apparently the thick border between 1-player and 2-player screens is *required* to make changes to the OAM (Object Attribute Memory). So the Neo Geo could be more flexible in this way - even if tricks are needed to get it to work.