Do you often say Twitter and texting together? Whenever you mention Twitter do you add texting as well? Do you say, "I was Twittering and texting"? Do you say, "kids these days with their Twitter and texting"? Or is it just in this one particular instance and usage that you consider them to represent a singular idea, and in every other usage of them together they wouldn't be used that way? If so, then I rest my case. :p

Because you can't actually argue that specific intent on a case by case basis should inform grammar. I mean, if I were to say, "My brother and sister has a plan", that would be incorrect. If I argue that I consider my brother and my sister to both be instrumental to a plan that they came up with together from start to finish and that they should be considered a singular subject insofar as it concerns the plan, that doesn't make it grammatically correct.