Is it possible to connect to two cameras with chdkptp and shoot simultaneously?
Yes, with some limitations. You can find some resources on https://chdk.fandom.com/wiki/Multiple_Cameras_using_CHDK
although much of it is oriented to larger rigs with many cameras and more stringent synchronization requirements.
For normal chdkptp CLI commands like shoot, remoteshoot and the file management commands, chdkptp only supports one "current" connection at a time. So to work with multiple cameras you can either use a chdkptp script like the included multicam.lua, run multiple instances of chdkptp, or use script to switch the current connection between cameras.
If you use multiple instances, you need connect to each one using bus/device ID, to ensure they don't step on each other's connections.
Also I would love to trigger the shots with a keyboard hot-key, like the spacebar, if that is possible.
In CLI mode, chdkptp only does line oriented input, so you could easily make a script that uses enter to trigger a shot, just using lua io.read(). If you script the GUI, you could respond to other keys.
Alternatively, you could do your keyboard input in another program, and control chdkptp either by piping to its input, or reading from your process using popen.