Grouping while sorting

Thank you very much in advance!
Ragulf Pickaxe

<?xml version="1.0">
<Root> <!-- Input 1 -->
 <Elem GroupID="A" Order="1">A1</Elem>
 <Elem GroupID="A" Order="2">A2</Elem>
 <Elem GroupID="B" Order="4">B4</Elem>
 <Elem GroupID="A" Order="5">A5</Elem>
 <Elem GroupID="C" Order="1">C1</Elem>
 <Elem GroupID="B" Order="7">A7</Elem>
</Root>

<?xml version="1.0">
<Root> <!-- Input 2 -->
 <Elem GroupID="A" Order="7">A7</Elem>
 <Elem GroupID="A" Order="5">A5</Elem>
 <Elem GroupID="B" Order="4">B4</Elem>
 <Elem GroupID="A" Order="2">A2</Elem>
 <Elem GroupID="C" Order="1">C1</Elem>
 <Elem GroupID="B" Order="1">A1</Elem>
</Root>

<?xml version="1.0">
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
 <Output>
   <xsl:apply-templates select="Root"/>
 </Output>
</xsl:template>

<xsl:template match="Root">
 <xsl:apply-templates select="Elem">
   <xsl:sort select="@GroupID">
   <xsl:sort select="@Order" data-type="number">
 </xsl:apply-templates>
</xsl:template>

<xsl:template match="Elem">
 <xsl:if test="not(preceding-sibling::Elem/@GroupID=current()/@GroupID)">
   <Group><xsl:value-of select="@GroupID"/></Group>
 </xsl:if>
 <xsl:value-of select="."/> -
</xsl:template>
</xsl:stylesheet>

Output from input 1:
<Output>
 <Group>A</Group>A1 - A2 - A5 - A7 -
 <Group>B</Group>B4 -
 <Group>C</Group>C1 -
</Output>

Output from input 2
<Output>
 A1 - A2 - A5 - <Group>A</Group>A7 -
 <Group>B</Group>B4 -
 <Group>C</Group>C1 -
</Output>