I have an XML file with multiple root elements
Well, your file is not an XML document, but (if you're lucky) it is an XML external general parsed entity, so you can parse it by creating a dummy XML document that references it as an entity. You need a skeletal document like this:
<!DOCTYPE orders [
<!ENTITY e SYSTEM "datafile.xml">
]>
<doc>&e;</doc>
I don't know what input you need for Access, but once you've got a document like this you can use XSLT to transform it into the desired format.