hi Amol Gaikwad,
In microflow, have a decision check based on your requirement like empty or based on dropdown 1 value change dropdown value 2 using change object
See the constrained by function described here: https://docs.mendix.com/refguide/reference-selector/#xpath-constraints
Or use an onchange micro or nanoflow that empties the second dropdown so the user needs to select a new value.
You can use onchange microflow to change the value and set it to empty. Even if the options are enumeration you can just enter "empty" in the value space and it will accept it as valid option.
I assume its an enumeration attribute then empty is a valid input to it.