Combining two encryption methods using separate keys can never be weaker than the inner encryption. Proof: if the combined method IS weaker, the attacker could just use the outer method as an attack with a small constant complexity on the inner method, which means the inner method is exactly as strong as the combined method (times a small constant). Unfortunately the same does not apply to the outer method, it is possible for a weak inner method to defeat the security of a strong outer method (e.g. if the inner method always starts its output with a known pattern, it can really mess up your outer method). Therefore, always encrypt FIRST with the algorithm you trust most, and NEVER use the same key for both methods.