Advice for cooking two pumpkin pies at once
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.