This thread has a script to run a zone for a specified number of seconds. Modify it to run multiple zones, or otherwise as needed for your application:
Hear, hear! IMHO, Rachio has a ‘style trumps substance’ culture and did extra work to remove the seconds option. If the duration fields for a schedule were simple text boxes where you type in the number of minutes, you could enter e.g. 0.2 to get a 12 second run. But they thought it was cute to have + and - buttons, which add or subtract a minute with each tap. Fine, but how about you also allow tapping on the number, where you could then type something in, including a decimal point if needed.