The variable "variable2" must be imported to the included view.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<!-- Include a file -->
<opt:root>
Foo{$variable1}{$variable2}
<opt:include view="$view" variable2="$variable2"/>
</opt:root>

>>>>templates/included.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<!-- Include a file -->
<opt:root>
Bar{$variable1}{$variable2}
</opt:root>

>>>>expected.txt
OUTPUT

>>>>result.txt
FooABBarCB
>>>>data.php
$view2 = new Opt_View('included.tpl');
$view2->variable1='C';
$view2->variable2='D';

$view->view = $view2;
$view->variable1='A';
$view->variable2='B';