I've seen this asked a lot, and when I answer I usually get someone trying to shoot me down in flames.
For the record, I'm an electrician so have a slight insight here.
TL;DR: Preference is separate cables, however the single "dual" cable is fine.
Explanation:
Logic tells you that the single cable is fine, because... well... they simply wouldn't provide it if it wasn't able to be used. It's an SLI cable, and is fine to use for 2 graphic cards with a single 8 or 6 pin connection, or indeed a single card with 2 connections.
Preference is to use 2 separate cables, each with a single 8 or 6 pin plug. The reason for this is that 2 separate cables are connected to the common rail in parallel TWICE at 2 separate points. The dual cable is connected to the common rail at a single point.
Now, if we assume that each cable is required to carry 10 amps then it is better to have each single separate cable pulling 10 amps individually back to the common rail. A dual cable will only have the amperage separated at the short link between the two plugs, and will then have to carry 20 amps along the shared cable back to the common rail. However, it does stand to reason that the shared cable will be rated for 20 amps else it wouldn't be provided in the dual configuration.
2 separate cables means less stress on the current carrying conductors, and that's always a good thing regardless of actual current rating. Heat reduces the current carrying capacity of cables, and some cases can be very hot (particularly SLI setups) so it's better practice to have the headroom.
The two connectors on a GPU aren't there to "give it more power". The card will consume the required power regardless of how many connections there are. The two connectors are there to give it more paths back to the PSU, and it's more to do with splitting the load on the PCB.
I have no idea why parallel and series circuits have been mentioned. The PSU cables are connected in parallel regardless of whether you use one dual cable or two separates.
HTH.
