Getting Upper XPath Value Based on Lower XML value

I'm trying to get the value of SERepeatKey if ItemOID REQUIRED.REQ_V has a value of "1". In the below case I want to return the value 2. Is this possible with this XML? If so how?

With this I was able to do it if there was only 1 "Data" node but now with 2 I am at a loss. In XPath tester I get the below error message.

string(ODM/Data/SData[@SKey='003-003'  and SiteRef[@LOID='SE 003']]/SEData[@SEOID='UNSCHEDULED']/@SERepeatKey)

"Unable to perform XPath operation. A sequence of more than one item is not allowed as the first argument of string() (@SERepeatKey, @SERepeatKey)"

<ODM>    
<Data SOID="(DEV)" MetaDataVersionOID=""  >
    <SData SKey="003-003" >
        <SiteRef LOID="SE 003"/>
        <SEData SEOID="UNSCHEDULED" SERepeatKey="1">
            <FData FOID="REQUIRED" FormRepeatKey="1">
                <IGData IGOID="REQUIRED" IGRepeatKey="0" TransactionType="Upsert">
                    <IData ItemOID="REQUIRED.REQ_V" Value="0"  />
                </IGData>
            </FData>
        </SEData>
    </SData>
</Data>
<Data SOID="(DEV)" MetaDataVersionOID="" >
    <SData SKey="003-003" >
        <SiteRef LOID="SE 003"/>
        <SEData SEOID="UNSCHEDULED" SERepeatKey="2">
            <FData FOID="REQUIRED" FormRepeatKey="1">
                <IGData IGOID="REQUIRED" IGRepeatKey="0" TransactionType="Upsert">
                    <IData ItemOID="REQUIRED.REQ_V" Value="1"  />
                </IGData>
            </FData>
        </SEData>
    </SData>
</Data>
<Data SOID="(DEV)" MetaDataVersionOID="" >
    <SData SKey="003-003" >
        <SiteRef LOID="SE 003"/>
        <SEData SEOID="UNSCHEDULED" SERepeatKey="3">
            <FData FOID="REQUIRED" FormRepeatKey="1">
                <IGData IGOID="REQUIRED" IGRepeatKey="0" TransactionType="Upsert">
                    <IData ItemOID="REQUIRED.REQ_V" Value="1"  />
                </IGData>
            </FData>
        </SEData>
    </SData>

This XPath should do what you are looking for:

//Data[.//IData[@ItemOID='REQUIRED.REQ_V' and @Value='1']]//SEData/@SERepeatKey

According to the presented XML looks like you can even omit @ItemOID='REQUIRED.REQ_V' and make this expression simpler

//Data[.//IData[@Value='1']]//SEData/@SERepeatKey