I think that it depends on how much of difference the 2 items make in comparison to the volume of your oven. A sheet pan with 2 chicken quarters will cook in the same time as a pan with 4, or maybe even 6. But if you fill 2 half sheet pans with 10 quarters each, put one on the top rack and one on the bottom rack, now you have made a significant difference in the total volume. It will take longer for the oven's heat source to heat that much volume of product to a proper temperature. The difference is not directly proportional either. It doesn't take 6 times as long to do 6 times as much.
The best method for accurately cooking most meats is with a good food thermometer. Know what internal temperature you need, then cook to that temperature. I wouldn't be without my Chef Alarm probe thermometer for doing any kind of roasted meat. I used it in the oven for our Christmas prime rib, and I used it the other day on my gas grill for smoking a pork shoulder roast. Stick the probe into the center of the thickest part of the cut, set the internal temperature you need, and it beeps with a loud alarm when it hits that point.
For everything else, I have my Thermapen instant read thermometer, another tool that I find indispensable.