Check the opt:include "view" attribute. It should parse an external view.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<!-- Include a view -->
<opt:root>
Foo
<opt:include view="$view" />
</opt:root>

>>>>templates/included.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<!-- Included file -->
<opt:root>
Bar
</opt:root>

>>>>expected.txt
OUTPUT

>>>>result.txt
FooBar
>>>>data.php
$view->view = new Opt_View('included.tpl');