You're close. Since you want to set the selected drop down after the first, you can't really use the Default Value. So for your first drop down, click the advanced option in the Drop DOwn form control, and you'll see an OnChange Client Script.
Set it to this:
$jq = (typeof $cmsj == "undefined" ? $ : $cmsj); $jq("select[id*='MySecondDropDownFieldName']").val($jq(this).val() == "Test" ? "1" : "Test This");
The first part is because inside the CMS there is no $, but there is $cmsj, so this just makes sure to grab either $ or $cmsj whichever is defined. Not totally necessary but without it this logic won't work within the CMS, only on the website portion.
The rest is pretty simple, it uses an ID wild card lookup to grab the 2nd drop down's select and uses this (the first drop down)'s value to set the other. Tested and worked for me.