The precision level of most power meters is not tight enough to catch the small changes you would be seeking to find.
Most PMs are +/-1 to 2% of the reported power.
That means one meter could read 198-202 watts at a true 200w at +/-1%.
That means one meter could read 196-204 watts at a true 200w at +/-2%.
Double each for the second PM and you get a max range wider that the watts you are trying to capture.
These are tools for training and lack the precision necessary to do a tighter level of review even approaching something scientific.