I would suggest a second chain as ideal since two different cassettes with different sets of cogs would also be ideal. Different sized cassettes call for different lengths of chain. Therefore two chains with a tool-free link such as an SRAM would be very beneficial. Yes, chains and cassettes do have a tendency to wear out together, but I can usually get 2-3 chain replacements before I have to change cassettes. Of course, that is only possible by closely watching your chain for stretching or stuck links. I clean and lube after every ride in the rain or mud (only takes 15 minutes), or at least monthly in dry weather.