Example with 1 router/ap: https://tangentsoft.com/mikrotik/wiki?name=Isolated%20Guest%20WiFi%20Sans%20VLANs
Forum Thread: http://forum.mikrotik.com/t/isolated-guest-wifi-sans-vlans/173913/1
It just works with 1 router/ap! It is mentioned with a VPN you could Tunnel your Guest traffic to your Gateway for multiple cap’s.