How to use pyserial to write two separate message?
If you want a quick, cheap, and easy solution, you could just use sleep() in your Python script to put a delay between the commands. This obviously isn't a perfect solution, but it's not difficult to implement.
Or, like you said, you could use a delimiter. It's not as easy to implement, but it'll work better.
If you go the delimiter rout, you'll probably need to use Serial.read() instead of Serial.readString(). That way you can examine characters individually as they come in. You can store your characters in a String or char array until you detect your delimiter and then execute your command.
Hope that helps!