If a recipe says to cook 1 lb of meat for 12 hours but I want to cook 2 lbs of meat. How do I adjust the time cooked?
Doubling the amount of meat in your bath doesn’t necessarily mean that you need to increase the cooking time at all - not unless you’re doubling the thickness of the package (if you split the meat into two packages, preserving the original thickness, then the cook time is the same).
Here’s a couple thickness guides (if you google “sous vide thickness chart” without the quotes, you’ll get a bunch of results).
Hope that helps! 
I have done a large bath with several pounds of steaks but since they were separate and the water was free to flow around them all it took the same amount of time 1 steak would have. A big roast is a different matter and the heat needs more time to penetrate to the middle.