One thing I wonder is if your oven temperature is accurate? An oven thermometer helps there. As well as a pizza stone to maintain a steady temperature. Also, how big was your chicken? Maybe you didn't cook long enough for the weight.
The method I use is this: Pre-heat the oven to 450. Clean bird, remove innards, truss. Salt (Tbls or so) and pepper or otherwise rub on marinade. Place the bird on a large cast iron skillet and cook for 1 hour. Remove bird and let rest for 10 minutes and meanwhile make a sauce utilizing the remnants in the pan. The skin is brown and the meat is juicy. This method is in Michael Ruhlman's, 'Ratio'. I find it to be a great method for basic roasted chicken.
I usually use a 4 - 5 lb chicken and haven't checked the internal temps since the first few times of using this recipe except for when I'm at or over the upper end.
talon8 is absolutely right: the best way to go about this is to use a thermometer. However, it is still an interesting question how the expected cooking time varies with the weight of a roast.
It is such an interesting question that in 1961, SIAM Review published a scholarly article entitled "On Cooking a Roast". To be a bit more precise, the question that the authors answer is this: Suppose we have two roasts with exactly the same combination of identical tissues, and the same shape except that the one is a scaled (blown up) version of the other. Let's weigh both roasts and compute the ratio between the two weights; let's call that ratio r. Suppose furthermore that the roasts have the same starting temperature, and we will cook both until they have both reached some given (higher) temperature at their centre. What can we say about the times that both roasts need to cook for?
The answer is that under these assumptions, the bigger roast will need r^(2/3) times as long as the smaller one.
So, assuming each small roast (700 and 950 grams) has the same shape as the big one that the recipe writers used (let's say 1750 grams), for the 950 gram roast you'd expect to use (950 / 1750)^(2/3) = 0.665 or about two-thirds of the cooking time, and for the 700 gram one you'd expect to use (700 / 1750)^(2/3) = 0.543 or just over half of the cooking time. In particular, for the 950 gram roast, you should expect to roast it at 200C for 20 minutes, then turn down to 180 and roast for around 0.665 * 1750 / 450 * 12.5 = 32 minutes more. (1750 / 450 * 12.5 = 49 minutes is the average time they would recommend for the 1750 gram roast, and we're using that as the base for comparing our 950 gram roast against.) Then check with your thermometer!
Finally, I think for the resting time, you might as well use the original 30 minutes, because that's not (only) about getting the internal temperature to a certain level (although it does contribute to that - the heat will spread out through the meat), but also about things like the muscle fibers relaxing and the like, which are not covered by the article.
Best Answer
Putting 2 pies in the oven will likely cool the oven more, and the oven will try to heat up to compensate. So its best to keep the temperature the same and cook longer. I'd say 5 minutes would be adequate in this scenario. Also, do not put the pies on different racks in the oven, the one on the lower rack will cook at a lower temperature and your product will be inconsistent.
If you have a convection oven, or a convection setting on your oven, use it. The movement of hot air will mitigate most of the effect of having another pie in the oven, its also more energy efficient and cooks more evenly. If you do not have a convection oven it might be wise to rotate the pies so they cook evenly. The pies will likely create a cold spot between them, in addition to any potential cold spots in your oven.
Another way of getting rid of cold spots and helping the oven retain more heat is to add a baking stone. I keep my baking stone in the oven all the time, because it helps hold onto heat, even if I'm not cooking directly on top of it. It helps hold heat when I open the over door, and it helps my oven heat more evenly. The trick is to let the oven preheat 5-10min longer before you put your goods in.