Based on the dropdown values, you want to redirect users to different page, right?
I consider you will button in the form. On clicking the button, call below function...
function redirect() {
if(document.getElementById('select_17').value == 'Ja' && document.getElementById('select_18').value == 'Ja') {
window.location.replace = 'both parents url';
} else if(document.getElementById('select_17').value != 'Ja' && document.getElementById('select_18').value != 'Ja') {
window.location.replace = 'both parents not coming url';
} else {
window.location.replace = 'one parents coming url';
}
}