There is an idea of one coin.. Myriadcoin. Good idea IMO !http://coingolem.com/myriadcoin-multiple-proof-of-work-algorithms-why-isnt-this-more-popular-327/
Theoretically - it is possible to build a coin, that has several algos, and switches between them.
If we have 2 algo:
Then even blocks could only be mined using first algo, and uneven blocks could only be mined using the second algo, making ASICs more difficult to build.
What do you think of it ?