The thing about Lynch is that he steadfastly refuses to explain his movies. He expects the viewer to craft their own unique experience from the films. That being said, there are a lot of good "interpretations" of these two movies out there. Just remember that all theories are "unofficial". They don't give you enough space here in the Answer section to fully answer your questions, but here are a couple of links:
This is probably the best take on "Mulholland Drive" I've seen, and most Lynch fans pretty much agree with it's conclusions:
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"Lost Highway" is a slipperier fish, so there are a few takes on it, but here are a couple of pages I've found useful:
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Finally, you can find a lot of knowledgeable Lynch fans here:
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Read through the Forums if you visit that last link. If you have questions, chances are someone's already started a topic on it.