Safe... probably yes. You could try it without damaging anything.
Will it work... probably no.
A GTX680 needs two 6 pin pci-e connectors, and your psu only has one.
Corsair is a solid brand, but if it had sufficient power to deliver 75w more to a second adapter, it would have done so.
Only like using adapters as a crutch till a proper unit can be purchased
Assuming ( oh no ) that the 450w mentioned is the VX;
Except for the fact that it is rated 1a lower on the 12v but uses better capacitors, it is identical to the original Seasonic built EA500, that did have 2 x 6 pcie conns.
Is also rated higher ( 33a v 30a ) than the Seasonic S12II 430 , that has 2 x pcie conns.