The overarching concept is that you want to get heat to the hardest-to-reach spot of whatever you are cooking (generally the center), and how long that takes is a function of the relative size and shape of what you’re cooking.
It takes much more time and energy to get heat to the center of a chunk of meat that weighs 7 pounds than it does to reach the center of something that is half its size and more of less the same shape. It takes much more time and energy to get heat to the center of a chunk of meat that weighs 7 pounds than it does to reach the center of two somethings that are half its size and more or less the same shape.
In fact, if one piece of meat is twice as thick as another (of the same cut), it will take four times as long to cook through as the smaller piece, because the heat has to travel twice the distance and the quantity of meat to be heated is doubled.
Theoretically you could cook forty 3-1/2-pound legs of lamb and you’d still be cooking them as if there were only one. (There are some other issues – if they’re touching each other, it takes more time for the heat to penetrate; if you put 40 refrigerator-temperature legs in the oven, it will take a lot longer for the oven to compensate for that much cold meat, etc.)
Similarly, if you put two 3-1/2-pound cold legs of lamb in the oven, it will take the oven a teensie bit longer to stabilize its temperature than if you put in only one cold 3-1/2-pound piece of meat. (Letting the roasts reach room temperature before putting them in the oven will help in this regard). But it will still take much less time than if you were cooking a 7-pound roast.
Now, having said all his, we’re going to take yet another opportunity to suggest that you rely on a meat thermometer, rather than the recommendations of a quite-possibly out-of-date recipe. Most cookbooks no longer give general cooking times, because they recognize that cuts of meat vary in size and shape, that some cuts have more or less fat, and that there is a lot of variation in how ovens heat. If you want to be sure that your leg of lamb is rare (140°F/60°C), medium (150°F/65°C) or well done (160°F/71°C), use a meat thermometer and take the roast out of the oven about 5 degrees before it reaches your desired temperature. The internal temperature will continue to rise during the 15 to 30 minutes that the meat rests while you get everything else ready to serve.