yes, try to follow these steps :
1. Edit "includes/view-functions.php" file and go to around line 864 for this code
$element_markup = <<<EOT
then add these code above that line
if ($_GET['id'] == 1 && $element->id == 2) {
$element->populated_value['element_'.$element->id.'_1']['default_value'] = '1';
$element->populated_value['element_'.$element->id.'_2']['default_value'] = '0';
$locked = ' readonly ' ;
}
2. Go to around line 872~879 for these code
<span>
<input id="element_{$element->id}_1" name="element_{$element->id}_1" class="element text currency" size="10" value="{$element->populated_value['element_'.$element->id.'_1']['default_value']}" type="text" /> .
<label for="element_{$element->id}_1">{$main_cur}</label>
</span>
<span>
<input id="element_{$element->id}_2" name="element_{$element->id}_2" class="element text" size="2" maxlength="2" value="{$element->populated_value['element_'.$element->id.'_2']['default_value']}" type="text" />
<label for="element_{$element->id}_2">{$child_cur}</label>
</span>
then change it to
<span>
<input {$locked} id="element_{$element->id}_1" name="element_{$element->id}_1" class="element text currency" size="10" value="{$element->populated_value['element_'.$element->id.'_1']['default_value']}" type="text" /> .
<label for="element_{$element->id}_1">{$main_cur}</label>
</span>
<span>
<input {$locked} id="element_{$element->id}_2" name="element_{$element->id}_2" class="element text" size="2" maxlength="2" value="{$element->populated_value['element_'.$element->id.'_2']['default_value']}" type="text" />
<label for="element_{$element->id}_2">{$child_cur}</label>
</span>
in there I assume your form id is 1 and the currency field id is 2. Don't forget to change those id's with yours