I don't think so.
It seems like you need some intervention by a server first,
to fetch the data from the first source and then the second source,
then mash the two together into a single data result set and send
that to the Flex app.
You can probably "construct" your single dataprovider inside
Flex as well, creating a new query or xml object, calling one data
source and then the other to construct its contents.
Never done it myself, tho. I tend to do as much as possible
on the server side, to keep the Flex app as light as