I would suggest looking at what is going to consume this "aggregation" you are trying to do and see if there are other options.
These two XML files use ( slightly ) different schemas, so you will never be able to just merge them together until you take a crash course on XML, namespaces and the XML DOM.
About the best you could do is create a PSCustomObject that represents a union of the data from each file.
Good Luck,
MJ