Can't state for sure any changes to treatment outcome splitting the Sprycel may induce (I'll leave that to Trey), but you'd need to weigh down to at least the 0.1 mg level to get decent accuracy. That gets expensive, at least $1.5K and easily up to $5K. I wouldn't trust one that goes to 1 mg level, not when you need to weigh out 70 mg. (As a chemist, I'm pretty picky on the accuracy. You might want to ask a pharmacist what type of scale they use. That would better indicate your need.) I'm sure you can find a scale online that purports to do that for under $500, but remember you get what you pay for.
Also, from the materials viewpoint, the coatings on tablets often protect the ingredients from moisture. Once exposed to air, some of the ingredients may get wet and sticky. That wouldn't be a problem for the first day's dose, but the 2nd day, after the powder has been sitting around awhile, it may be difficult to handle. Speculation on my part, don't know that Sprycel will get sticky on air exposure, but it could. Lots of drug compounds do, at least the ones I've handled in the lab. We sell the research quantities as powders, and many are stored and packaged with dessicant. Of course, if you can dissolve the sticky powder in water and drink it, then the potential stickiness is not an issue.
P.S. I still like your idea to alternate 50 and 100 mg dose, although I don't know if that works well when one considers the half-life of the drug. If the insurance company refuses to fill the 70 mg until the 100 mg 90-day supply is used up, then that would be the option I'd push with the doctor. You've got to do what you can afford to do.