I imagine you’re also adding a filter mce_buttons
to add in the button to, something like..
add_filter( 'mce_buttons', 'add_my_tinymce_buttons' );
function add_my_tinymce_buttons( $items ) {
$items[] = 'your-button';
return $items;
}
Just change the filter to hook onto mce_buttons_2
instead, and the button will appear on the second row , eg..
add_filter( 'mce_buttons_2', 'add_my_tinymce_buttons' );
Hope that helps.. 🙂