1. A method to provide fault tolerance in a virtualized computing environment with a first fault domain and a second fault domain, the method comprising: determining whether a primary virtualized computing instance and a secondary virtualized computing instance are both in the first fault domain, wherein the secondary virtualized computing instance is configured as a backup for the primary virtualized computing instance and supported by a first host; in response to determination that the primary virtualized computing instance and secondary virtualized computing instance are both in the first fault domain, determining a resource availability of each of a plurality of hosts that include the first host and the second host, wherein the second host is in the second fault domain; comparing the resource availability of each of the plurality of hosts to select the second host with a most resource availability; based on a resource consumption of the secondary virtualized computing instance, updating a resource availability of the first host and a resource availability of the second host; and migrating the secondary virtualized computing instance from the first host to the second host based on the updated resource availability of the first host and the updated resource availability of the second host, thereby migrating the secondary virtualized computing instance from the first fault domain to the second fault domain.