If you want to have just an additional Style to choose from for your CQWP you should not add another ItemStyle.xsl, but try to add another xsl-template to the existing itemstyle.xsl (make a backup before playing with the original one.

You just have to copy&paste an existing template like

<xsl:template name="TitleOnly" match="Row[@Style='TitleOnly']" mode="itemstyle">
…
</xsl:template>

And Change the name and @Style value to fit your new View.

Then Upload it to the \Style Library\ItemStyle.xsl Afterwards you can choose this new format in the settings of the cqwp.

You can also change the Layout of your headers with this way. A more detailed description can be found in an MSDN article

There is also the possibility to assign a itemstyle.xsl to just one cqwp with the help of the SharePoint Designer but I wouldn’t recommend that.

If you want to have just an additional Style to choose from for your CQWP you should not add another ItemStyle.xsl, but try to add another xsl-template to the existing itemstyle.xsl (make a backup before playing with the original one.

You just have to copy&paste an existing template like

<xsl:template name="TitleOnly" match="Row[@Style='TitleOnly']" mode="itemstyle">
…
</xsl:template>

And Change the name and @Style value to fit your new View.

Then Upload it to the \Style Library\ItemStyle.xsl Afterwards you can choose this new format in the settings of the cqwp.

You can also change the Layout of your headers with this way. A more detailed description can be found in an MSDN article

There is also the possibility to assign a itemstyle.xsl to just one cqwp with the help of the SharePoint Designer or Download and re-upload the cqwp but I wouldn’t recommend that.