I havent found anything that says not however if you look here
http://www.w3.org/TR/html4/present/styles.html the author has particularly used seperate link statements to link external stylesheet.
http://www.w3.org/TR/REC-CSS1
CSS style sheets are often embedded in HTML documents, and to be able to hide style sheets from older UAs, it is convenient put the style sheets inside HTML comments. The HTML comment tokens "<!--" and "-->" may occur before, after, and in between the statements. They may have whitespace around them.
At-rules start with an at-keyword, which is an identifier with an '@' at the start (for example: '@import', '@page'). An identifier consists of letters, digits, dashes and escaped characters (defined below).
An at-rule consists of everything up to and including the next semicolon (;) or the next block (defined shortly), whichever comes first. A CSS1 UA that encounters an at-rule that starts with an at-keyword other than '@import' ignores the whole of the at-rule and continue parsing after it. It also ignores any at-rule that starts with '@import' if it doesn't occur at the top of the style sheet, i.e., if it occurs after any rules (even ignored rules). Here is an example.
Assume a CSS1 parser encounters this style sheet:
@import "subs.css";
H1 { color: blue }
@import "list.css";
The second '@import' is illegal according to CSS1. The CSS1 parser skips the whole at-rule, effectively reducing the style sheet to:
@import "subs.css";
H1 {color: blue}
May be this a long time misunderstanding that i may have developed, that you cant link one stylesheet with another compliant with W3C
Please Mark Post that helped you as answer, also include a summary of what solved the problem as it helps others in similar situations