It is not a bad idea, but not a good one wither.
I advocate using a single great card vs. two where one card will do the job.
If you are planning on sli/cf, you must pay more up front for a sli/cf capable mobo, and a stronger psu.
The savings using a single card will pay for a stronger video card up front.
The 28nm cards are due out the end of the year. You can expect them to run cooler, faster, and be cheaper to produce.
At that time, it will be unattractive to pay for a second obsolete card to sli/cf. The prices don't come down that much as you might expect.
Perhaps because of the sli/cf demand.
It is simpler to just sell the old card on ebay and replace it with something much stronger.