Still works, with a slight variation: instead of taking two measurements, you can take four successive measurements [A,B,C,D] over equal periods of time, and th

Still works, with a slight variation: instead of taking two measurements, you can take four successive measurements [A,B,C,D] over equal periods of time, and then do the aforementioned procedure with the values A+D and B+C as input. If the activity is decreasing at a linear rate, then A+D and B+C should have exactly the same distribution. In reality, they will be ever so slightly different because the decay is exponential rather than linear, but over short periods of time (compared to the half-life) the linear approximation is extremely good.