I have a pill cutter and a milligram scale. However, my experience is that it can be difficult to accurately cut small coated pills like these. I personally would dick around with it, or go to the generic sooner, but for my SO I'd rather keep it as simple and safe as possible while we see how this plays out.
Interestingly, the EINSTEIN CHOICE study found that 10 mg was more effective than 20 mg for preventing VTE recurrence, as well as having a lower incidence of bleeds (but not at a level of statistical significance), which would be expected.
https://www.nejm.org/doi/full/10.1056/NEJMoa1700518
Rivaroxaban or Aspirin for Extended Treatment of Venous Thromboembolism
Jeffrey I. Weitz, M.D., et al, March 30, 2017, N Engl J Med 2017;376:1211-1222,DOI: 10.1056/NEJMoa1700518,VOL. 376 NO. 13