I don't think you'd want to manually bring up a new DHCP server when your primary is down. the binding database wouldn't be synchronized so the new server would have no way to know what address are already in use. Most modern DHCP servers try and arp/ping for an address before they hand it out just to be safe... Not sure if the Meraki devices do this since I haven't checked. Maybe someone else knows?
IMO If you want redundant DHCP servers you're better off to use a dedicated DHCP product that is built to properly handle redundant servers.