While they use the same architecture they are different gpus, hence the different model numbers.
They are actually the same GPU but the 660Ti has a crippled unit which is why the bandwidth is cut down to 192 bit.
^While you technically might be able to, it's a silly idea - sli will make sure that the cards are running exactly the same; meaning the 670 will be downclocked and have cores disabled so that it has the specs of a 660ti. (So it's pointless to go that route.)
And that's why you might have a problem or several to get them to SLi, technically.