You do not necessarily have to use Advanced XML for this at least in Splunk v6.0. Here's my example of how device_type selected with radio button is passed to the search populating dropdown. In two words: just add in the beginning of populating search your previous value like device_type=$device_type$
<fieldset autoRun="true" submitButton="false">
<input type="time" searchWhenChanged="true" token="cohort"> <default>Last 30 days</default> </input>
<input type="time" searchWhenChanged="true" token="search"> <default>Last 30 days</default> </input>
<!-- Specify search when changed for the inputs -->
<input type="radio" token="device_type" searchWhenChanged="true">
<label>Select a device type:</label>
<default>All</default>
<!-- Define the choices in code -->
<choice value="*">All</choice>
<choice value="TV">TV</choice>
<choice value="BDP">BDP</choice>
<choice value="WEB">WEB</choice>
<choice value="TBL">TBL</choice>
<choice value="MOB">MOB</choice>
</input>
<input type="dropdown" token="device_manufacturer" searchWhenChanged="true">
<label>Select a device manfacturer:</label>
<prefix>device_manufacturer="</prefix>
<suffix>"</suffix>
<default>*</default>
<!-- Define the default value -->
<choice value="*">All</choice>
<!-- Define the choices with a populating search -->
<populatingSearch fieldForValue="device_manufacturer" fieldForLabel="device_manufacturer" earliest="-48h" latest="now">
<![CDATA[source=sc_context_creation device_type="$device_type$" | eval device_manufacturer=lower(device_manufacturer)
| stats count by device_manufacturer | sort 0 - count]]>
</populatingSearch>
</input>