POSSIBLE SOLUTION(S)
We use the data to determine the rate constants k1 and k2.
We formulate a least squares function in the constants (now variables) k1 and k2 using the data from the chemicals -- first separately and then together.
The rate constants k1 and k2 seem to differ depending upon which of the data sets we wish to "satisfy." Thus we take the values which satisfy "all" of the data.