Hello dear, How can I do so that only one of the routes to be removed, if two link health monitor configured in a single interface with two default routes? Here topology.
As far as I have understood DGD will remove all routes associated with a port if the port's ping target fails. This will not only remove default routes.
One possible solution to this would be to use 2 WAN ports. Note that no 2 ports can be part of the same network though.
You can also try dynamic routing between your fortigate and the ISP routers. This would dynamically remove one route in separate each time a link failure is detected. OSPF could be the functionality that best fits what you want to achieve.