It is very important that the temperature of the two cells stays the same during an experiment. As you can see from the above graph, there is a big delay if the cell temperatures differ, before they come back to thermal equilibrium.

Thus if any component is to be added to only one cell, it must either be matched with an equivalent addition to the other one [at the same temperature] OR the solution to be added must be brought to the correct temperature before addition. This is fairly easy to achieve if a small volume is added. I often use 0.01 ml [for the enzyme addition to one cell]. It is best to try a few control experiments before commencing a series of assays.

The exactness of the temperature balance becomes more important as the sensitivity of the experiment increases. Thus measuring mM changes as initial rates requires less attention to temperature balance than does measurements of 10 micromolar changes.