Super simple to to start Recording in OBS Studio, then go start the other application (this avoids risk of missing somethign as OBS Studio starts the Recording process)
- you can always use something like Avidemux to trim a video file WITHOUT quality loss of re-encoding ... easy peasy
Other options would be to configure and use a Hotkey, or a plugin (like Advanced Scene Switcher with some automation, which is what I use)
Other folks use something like a Stream Deck, basically an extra keypad interface with 'macro'-type actions associated with each button
