The reason, of course, is that we're talking dual channel vs. quad channel. What's the difference? Absolutely nothing. This is just a marketing thing.
Generally, two DC kits can work together to achieve QC architecture.
Be wary though, some of the manufacturer's tech support will not RMA your RAM if you buy a kit (or two) for other than its labeled purpose. I've seen a lot of threads here where two kits wouldn't play nicely together and the consumer(s) couldn't get anything from the manufacturer.
The savings of the two 4GB kits vs the one 8GB kit is what, 20 bucks? If you're that frugal, I'd e-mail the manufacturer tech support, before purchasing, to get their "official" answer of whether or not using the two kits together will produce the results you desire.