I believe one way to solve it is to use muenchian grouping, but in this case you are grouping by task and by person, and so you need an aggregated key
<xsl:key name="PersonTasks" match="workTime" use="concat(@taskID, ../name)"/>
This key is used to look up all distinct 'person tasks'. You would iterate over this by doing the following
<xsl:apply-templates select="//workTime[generate-id() = generate-id(key('PersonTasks',concat(@taskID, ../name))[1])]"/>
Then, for each distinct Person and TaskID, you just need to find all the work times. Try this stylesheet (Note I have added a 'People' element, to give the output a single root element to ensure it is valid XML)
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:key name="PersonTasks" match="workTime" use="concat(@taskID, ../name)"/>
<xsl:template match="/">
<People>
<xsl:apply-templates select="//workTime[generate-id() = generate-id(key('PersonTasks',concat(@taskID, ../name))[1])]"/>
</People>
</xsl:template>
<xsl:template match="workTime">
<xsl:variable name="taskID">
<xsl:value-of select="@taskID"/>
</xsl:variable>
<xsl:variable name="name">
<xsl:value-of select="../name"/>
</xsl:variable>
<Person>
<name>
<xsl:value-of select="$name"/>
</name>
<taskID>
<xsl:value-of select="$taskID"/>
</taskID>
<xsl:for-each select="//workTime[../name = $name][@taskID = $taskID]">
<workTime>
<date>
<xsl:value-of select="../date"/>
</date>
<time>
<xsl:value-of select="."/>
</time>
</workTime>
</xsl:for-each>
</Person>
</xsl:template>
</xsl:stylesheet>
When you apply this to the following input XML
<People>
<Person>
<name>John</name>
<date>June12</date>
<workTime taskID="1">34</workTime>
<workTime taskID="2">12</workTime>
</Person>
<Person>
<name>John</name>
<date>June13</date>
<workTime taskID="1">21</workTime>
<workTime taskID="2">11</workTime>
</Person>
</People>
You should get the following output
<People>
<Person>
<name>John</name>
<taskID>1</taskID>
<workTime>
<date>June12</date>
<time>34</time>
</workTime>
<workTime>
<date>June13</date>
<time>21</time>
</workTime>
</Person>
<Person>
<name>John</name>
<taskID>2</taskID>
<workTime>
<date>June12</date>
<time>12</time>
</workTime>
<workTime>
<date>June13</date>
<time>11</time>
</workTime>
</Person>
</People>