Mycelium autogenerates new addresses when the previous address is "used" (ie. it receives funds). This is to prevent "address re-use" as it is generally considered to be a "Bad Thing"™... generating new address for each transaction "helps" with privacy for yourself and others sending/receiving coins to/from you.
Anyway, this is why you see "contains 2 private keys"... you have the private key for the address in the "Output" section (this was the 1st private key in your Mycelium wallet). When that address received your initial deposit, Mycelium automatically generated the next private key and matching address.
If you send coins to the current address that shows in your balance (or receive) tab... you will find that Mycelium will generate a 3rd private key... and the address showing on your balance (and receive) tab will change again.
This will occur whenever the most recently generated address receives coins for the first time. If you send more coins to the 1st address, Mycelium will not generate another address.
Not actually
considered to be a "Bad Thing nor it is for security but rather than for
privacy thats is people will not be able to snoop regarding your balance(s).
To OP,
This only happens when you use an HD wallet. You can try researching the difference between HD wallet and classic wallet though the HD wallet has just been explained above and classic wallet just having a single address. You read it right, HD wallet is comprised of multiple addresses.
