<?php
$output = '';
$value1 = $modx->resource->getTVValue('singleRoom');
$value2 = $modx->resource->getTVValue('doubleRoom');
if ($value1 == 0 && $value2 == 0) {
$output = $modx->getChunk('AllFull');
} else
$sum = $value1 + $value2;
$modx->setPlaceholder('TotalRooms', $_POST['TotalRooms']);
$output = $modx->getChunk('AvailableRooms', $TotalRooms);
return $output;This question has been answered by muzzstick. See the first response.
$otherResourceId = '521'; // for example
$otherResource = $modx->getObject('modResource',$otherResourceId);
$value1 = $otherResource->getTVValue('singleRoom');
$value2 = $otherResource->getTVValue('doubleRoom');