Are those two 8 Pin connectors (6+2) like it is mentioned in the Specs for your Sonnet Breakaway: https://www.sonnettech.com/product/egfx-breakaway-box/techspecs.html#techspecs
Internal Connectors
- Two 8-pin (6+2 pin) power connectors
Those sound like the same type of GPU PCIe Power connectors that come with PSUs.
You can connect either just the 6 pin connector if the GPU card has a 6 PIN PCIe Power port or your can use the entire 8 Pins (6+2) to connect to your 8 Pin GPU PCIe Power Port.
This SuperUser Tech site explains it better:
What are PCIe 6 2 pin connectors for?
Some video cards have 6 Pin PCI Express power connectors and others have 8 Pin PCI Express power connectors. Many power supplies come with a 6+2 PCI Express power cable which is compatible with both kinds of video cards. The 6+2 PCI Express power cable is made up of two pieces: a 6 pin piece, and a 2 pin piece.
Here is a picture of a PCIe 8 pin (6+2) GPU Power connector: