Yes: half the baking soda is half the dKH. Do you make your dKH solution by diluting a large quantity of high dKH, which will give more accuracy? Example; making a 20 dKH solution in 3000 ml and diluting it down to a 2 dKH solution. The lower (smaller) you go, e.g.; from 4 to 2 dKH, the more difficult it is to be accurate.
The color change would still show correctly (remember that these color changes only approximate ppm levels). However, if you look at the standard pH/KH/CO2 color charts, you will see that, at 2 dKH, the ~15 ppm area of CO2 is just over the blue line into the lower end of the green region. So, you will want to adjust CO2 to catch that sweet spot where it just changes from blue to green. That will take some playing around because it will take 3-4 hours for a drop checker to respond to CO2 changes. Even then, you are probably going to be dealing with a +/- 2-3 ppm tolerance.
You should change your solution every week or so. I change mine every other week. They can get stale and will tend to not change color as readily when the CO2 drops. I use small (2-3 ml) pipettes (100 for $5-$10 on Amazon) to flush them clean with distilled water before refilling.