Usually when a modular PSU is labelled this way it means that, the socket on the PSU can be used for PCI-e 6+2-pin or 12VEPS 4+4-pin (CPU), but there's a different modular cable supplied with the PSU for each type connection.
So one output socket, but two different cables, depending on what you want to connect.
EDIT: The cable would be wired specifically to match either a 12vEPS socket or a 6+2-pin PCI-e connector, which is why there's 2 different cables for one output on the PSU. Like you mentioned yourself, an 8-pin CPU socket is not compatible with / wired correctly for a 6+2-pin PCI-e connector.
You can use that kind of PSU for the build described in your post, but you should make sure the PSU comes with 2x 4+4-pin 12vEPS / CPU cables, before deciding on a specific model.
Modular PSUs and cables are not a standard design, they can have different pin-outs on the PSU itself and the wires are then built to match the standard PCI-e 6-pin or 8-pin socket, or the standard 12v EPS, so they are not interchangable between all makes and models (unless it is specifically confirmed by the manufacturer).
EDIT : In other words. Modular cables can sometimes have varying pin-layouts on the end that connects to the PSU, but always a standard pin-layout on the ends that connect to the motherboard, GPU or harddrives. This is also the reason why the cables are not always interchangable between different makes and models
(For example, I have a custom cable kit made for a Corsair branded PSU built by Seasonic, but the cables are not compatible with my new Seasonic PSU, despite being designed and built by the same manufacturer)
So you should make sure all the required cables comes with the PSUs accessories.