Magento2 : Display two table column values in single UI grid column

Little late on the answer, but hopefully helpful to those that finds this question.

Vendor/Module/view/adminhtml/ui_component/customer_listing.xml

<column name="fullname" class="Vendor\Module\Ui\Component\Listing\Column\FullName">
    <argument name="data" xsi:type="array">
        <item name="config" xsi:type="array">
            <!-- You can change this accordingly -->
            <item name="sortable" xsi:type="boolean">false</item>
            <item name="dataType" xsi:type="string">text</item>
            <item name="bodyTmpl" xsi:type="string">ui/grid/cells/html</item>
            <item name="label" xsi:type="string" translate="true">Full Name</item>
        </item>
    </argument>
</column>

Vendor/Module/Ui/Component/Listing/Column/Fullname.php

namespace Vendor\Module\Ui\Component\Listing\Column;

class FullName extends \Magento\Ui\Component\Listing\Columns\Column
{
    /**
     * @param \Magento\Framework\View\Element\UiComponent\ContextInterface $context
     * @param \Magento\Framework\View\Element\UiComponentFactory $uiComponentFactory
     * @param array $components = []
     * @param array $data = []
     */
    public function __construct(
        \Magento\Framework\View\Element\UiComponent\ContextInterface $context,
        \Magento\Framework\View\Element\UiComponentFactory $uiComponentFactory,
        array $components = [],
        array $data = []
    ){
        parent::__construct($context, $uiComponentFactory, $components, $data);
    }

    public function prepareDataSource(array $dataSource)
    {
        if(isset($dataSource['data']['items'])){
            foreach($dataSource['data']['items'] as &$item){
                $item['fullname'] = $item['firstname'] . ' ' . $item['lastname'];

            }
        }

        return $dataSource;
    }
}

What is going on is that you are looping through all of the records for the grid listing, and applying data to the fullname column of the grid with the data firstname and lastname fields of your model.

Suppose your UI component column like:

<column name="customer_name" class="Vendor\Module\Ui\Component\Listing\Column\Customer">
    <argument name="data" xsi:type="array">
        <item name="config" xsi:type="array">
            <item name="sortable" xsi:type="boolean">false</item>
            <item name="dataType" xsi:type="string">text</item>
            <item name="bodyTmpl" xsi:type="string">ui/grid/cells/html</item>
            <item name="label" xsi:type="string" translate="true">Customer Name</item>
            <item name="sortOrder" xsi:type="number">20</item>
        </item>
    </argument>
</column>

Now create renderer class Customer[Vendor\Module\Ui\Component\Listing\Column\Customer]

namespace Vendor\Module\Ui\Component\Listing\Column;

class Customer extends \Magento\Ui\Component\Listing\Columns\Column
{

    /**
     * Get data
     *
     * @param array $item
     * @return string
     */
    protected function prepareItem(array $item)
    {
        // make your logic and return string

        return $customerName;
    }
}