Do multi-PoW currencies open up additional attack vectors on a blockchain? I'm curious if there have been any technical evaluations of the vulnerabilities of cryptocurrencies that utilize more than one proof of work algorithm - for instance, myriadcoin or huntercoin.

Myriadcoin uses five different PoW algorithms, with different algorithms targeted towards different hardware, so that people are able to mine with ASICs, GPUs, and CPUs. The idea is that this will make it more difficult to centralize mining and harder gain control of 51% of the hashing power, while also making mining more accessible to as many people as possible.

I'm wondering if there are any potentially unintended consequences to this that maybe some folks have already identified.
Chaining algorithms reduces security as if one part of the chain fails, the whole chain fails. And in this case fail doesn't just mean completely broken, it can also mean an ASIC-proof algorithm turning out to be not ASIC proof.