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You might have to do it with jQuery. On change for dropdown 2, select "function", and in the target field (the one on the right, so not the 'value' field) put the name of your function.
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In a custom JS block, put something along the lines of
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function myFunction(elem) {
   var drop1 = jQuery('select [name="dropdown1"]').dropdown('get value');
   var drop2 = jQuery('select [name="dropdown2"]').dropdown('get value');
   if(whatever your logic is) {
      jQuery('input[name="fieldname"]').removeClass("hidden"); // Will SHOW the field
   }
   else {
      jQuery('input[name="fieldname"]').addClass("hidden"); // Will HIDE the field
   }
}

If you need to do more complicated things like removing validation then you'll have to look at form validation in the Semantic UI docs.
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Alternatively, you can use a Segment or Container area, and onchange for the dropdown reload the segment/container area. Inside the area you would have a Switcher Area with the necessary fields under the necessary condition blocks.