I don't know of any simple way to do that. Whenever you ask MODX for the value of a TV, you get the one selected in the Manager (or the default, or inherited value if any).
This is off the top of my head, but I think what you want is this:
<?php
$id = 12; // ID of the TV
$tv = $modx->getObject('modTemplateVar', $id);
$values = $tv->get('elements') // If the values you want are in the 'element' field in the DB
// or
$values = $tv->getProperties(); // If they're in the 'properties' field.
In the first case, you'd need to parse the values yourself. In the second, they should already be converted to a PHP array for you.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html