@Balu

I might try something like this (note, it’s not been tested):

if(a.Field_One__c == null || a.Field_Two__c == null) {
    if(a.Field_One__c == null) {
        a.Field_One__c.addError('Field one was not completed.');
    }
    if(a.Field_Two__c == null) {
        a.Field_Two__c.addError('Field two was not completed.');
    }
    return null;
}
else {
    return std.save();
}

This should (in theory) attach the field error to each field separately.