It shows up to the OS as two 12GB cards still. So it kind of depends on whether the higher cost is worth the extra slot (in my case it isn't). Total power is less than 2X on the card versus one K40, although I'm not sure offhand if that indicates less performance or just power savings from shared components.
As mentioned in a peer comment, K40 aren't the fastest cards either, but the 12GB is really nice for some use cases.
I have one K40 as well; you should be fine, even Haas F1 team built a CFD supercomputer full of them fairly recently ;-) Kepler is pretty good in FP64.
As mentioned in a peer comment, K40 aren't the fastest cards either, but the 12GB is really nice for some use cases.