(1) If a document isn't well-formed, then it won't validate against any schema. A document will multiple root elements isn't well-formed.
(2) You can't use XSD to impose purely lexical constraints on your XML, for example to restrict the amount of whitespace between attributes, or the choice of single-or-double quotes, or the use of decimal-versus-hexadecimal character references, or the choice between <a></a>
versus <a/>
to represent an empty element. Receiving applications aren't supposed to care about these differences, and if they use a conforming XML parser they will never know about the differences anyway. XSD is for validating the logical structure, the stuff that applications do care about.