What type of attack are you trying to prevent?
If it's a brute-force attack, AES-128 is more than sufficient. In the best case scenario, combining RC4 and AES gains you negligible additional security due to a meet-in-the-middle attack.
Are you trying to hedge against a "break" of either RC4 or AES? If so, in the real world, this is extremely unlikely to happen. If either one is weakened by a cryptanalytic result, you will have plenty of time to migrate away from the affected cipher — and you should do this regardless of if you're encrypting only with this cipher, or encrypting with two separate ciphers.
So, practically speaking, there's very little upside. On the downside, you're now heavily increasing the likelihood that you implement the cryptography incorrectly. It's infinitely more likely that you will, as an amateur cryptographer, generate keys insecurely, store keys insecurely, generate two keys that are not independent of one another, use an insecure mode of encryption (e.g., ECB), use static IVs, generate IVs that don't satisfy the requirements of the cryptographic mode for which they're used, fail to use authenticated encryption (e.g., GCM / EAX modes), introduce timing attacks when creating your own authenticated mode (e.g., CTR + HMAC), and so on. There is a near infinite number of ways in which even experienced cryptographers can fail when working directly with ciphers, and well-intentioned amateur cryptographers have practically no hope of implementing it correctly.
The general advice here is to use a high-level cryptographic library like NaCl or KeyCzar that handles the details for you. If you're typing the letters A-E-S into your code, you're doing it wrong.