The scratchpad is part of the memory map. The advantage of the scratchpad over the memory map is that there are built in commands to read and write to them so you don’t need to know the memory map addresses and use the memory map commands (you still could if you wanted).
Unfortunately, strategy variables are not in the memory map, so a different controller can not read these directly (other than through the new REST API). If you are running 9.5 firmware and software, you probably want to look into the REST API for sure. If you are not using 9.5, then keep on reading.
The typical scenario is to have a chart on one of the controllers write the variables to the scratchpad that the other controller needs access to and have the other controller read them. You could use the scratchpad on either controller, it doesn’t really matter - just pick a convention and stick to it. I personally put the variables in the scratchpad of the controller that needs to be read from. If I was needed two-way communication, then I would use both scratchpads. Just be sure to document which scratchpad indexes are in use somewhere - I like to put a comment in my power up chart.
This would be a good post to read: Your better Peer-to-Peer for PAC - #11 by philip