Parameter causing wrong file name to be added to package file on epub3

Good morning:

I am using the following parameter to add 2 stylesheets to my epub book:

<xsl:param name="html.stylesheet" select="'carlos-new.css fixed-layout.css'"/> 

However it is causing the following to be added to the package file:

<item media-type="text/css" id="html-css" href="carlos-new.css fixed-layout.css"/>

My expectation is that the list of files would be broken in 2 and 2 separate items would be added to the package:

<item media-type="text/css" id="html-css" href="carlos-new.css"/>
<item media-type="text/css" id="html-css2" href="fixed-layout.css"/>
 
Am I adding the stylesheets correctly or do I need to change the stylesheet parameter?