After consulting the appropriate core file cross group depends looks like this:

<!-- … -->
<hello_message>
    <label>Message</label>
    <frontend_type>text</frontend_type>
    <sort_order>1</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
    <depends>
        <enabled>
            <fieldset>notes</fieldset>
            <value>1</value>
        </enabled>
    </depends>
</hello_message>
<!-- … -->

After consulting the appropriate core file a cross group/fieldset <depends> looks like this:

<!-- … -->
<hello_message>
    <label>Message</label>
    <frontend_type>text</frontend_type>
    <sort_order>1</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
    <depends>
        <enabled>
            <fieldset>notes</fieldset>
            <value>1</value>
        </enabled>
    </depends>
</hello_message>
<!-- … -->