XSL copy template and add new attribute

The input XML:

<a>
    <b1>
        <c1 width="500" height="200" />
    </b1>
    <b2 />
</a>

I want to copy all of the attributes from b1/c1 to b2/c1 AND add a new attribute (length). The output XML should be:

<a>
    <b1>
        <c1 width="500" height="200" />
    </b1>
    <b2>
        <c1 width="500" height="200" length="2" />
    </b2>
</a>

I have a code, which copy all from b1/c1 to b2/c2 BUT without adding a new atttribute (length):

<xsl:template match="/a/b2">
    <xsl:copy>
        <xsl:apply-templates select="@*" />
        <xsl:copy-of select="/a/b1/c1" />
        <xsl:apply-templates select="*" />
    </xsl:copy>
</xsl:template>

And I tried to add attribute to the copy part, but it doesn't work:

<xsl:template match="/a/b2">
    <xsl:copy>
        <xsl:apply-templates select="@*" />
        <xsl:copy-of select="/a/b1/c1" >
            <xsl:attribute name="length">2</xsl:attribute>
        </xsl:copy-of>
        <xsl:apply-templates select="*" />
    </xsl:copy>
</xsl:template>