Because you're working with a greater volume of meat, when you put it in, it will take longer for the smoker to get back up to working temperature, and it may be quite a while before the overall temperature in your smoker is where it needs to be. If your smoker doesn't auto-adjust its cooking temperature (turn itself up temporarily), then you can assume it will take longer to cook both loins properly.
Increasing the temperature of the cooking overall can help with this, but you do run the risk of making the outsides of the loins more cooked than you want.
One of the advantages of low and slow cooking is that the meat, even a large cut, can be more uniform in temperature throughout, rather than especially done on the outside and rare in the middle. If you turn up your temperature to make up for the extra meat, you will be casting some of the low and slow advantage aside.
My choice would be to use the same temperature you did before, but cook it longer. You'll eventually make up for the extra thermal mass of the meat, and you'll still have the advantages of slower cooking.
As Cos Callis said, you'll do well to use the internal temperature of the meat as your guide to how much longer you should cook.
Rule number 1 of smoking/barbecue: Don't try to make the meat come out at a specific time. It needs as long as it needs, and there's very little you can or should do about it. If you're on a deadline, start early enough that you're certain to be early with the meat, then hold it at temperature. Much better results than rushing it.