I have two Schengen visas, both multiple-entry, both issues by the Netherlands embassy. One expires on the 30th of September and the other start October 1st. I want to be in Paris from 30th Oct-2Oct. Is it OK to travel?
Your first Schengen will expire at 23:59 on 30 Sept. Your second will begin at 24:00 on 1 Oct. They are the same instant and you are legal throughout. You can use the Schengen calculator to determine if 1 Oct counts as a day of leave or as an 'arrival' day and how that will affect your overall spent leave in the Schengen zone. But for your question: yes, you are ok.