Doubling a recipe: how much oil for browning, sauteing?

The main issue I would be wary of is that you have sufficient space in the pan for all of the meat to brown well.

Generally, it's better to do the meat in batches so that it has room and doesn't cool the pan off too much - you want to brown the meat, not steam it. When I make stew in my stainless steel 12 inch pan using two pounds of stew meat (cubed beef, lamb, or bison), I still usually need to do 2-3 batches to leave room for good browning.

So, while this is a tangential answer, my recommendation is to do one recipe worth of the meat at a time (if your pan can accommodate it all) and, with the second batch, add enough oil to make it look like the same amount of oil as the first batch had. While the meat may not absorb much, the flour may, and the meat will get coated - plus there's usually a good amount of splatter, so you will likely need to add at least 2-3 tsp of oil for the second batch.

As to the onion, I'd probably increase it by half again, so use 3 tsp. Since you're doing this all in the same pan the important thing is to really look at what's in the pan before you add oil each time. If there's still a lot of fat in the pan after you've finished your browning, use less oil. Alternately, if you're using a really fatty cut of meat, it may release a bunch of grease, so you don't really need to add a lot of oil. I find that, quite often, the amounts of oil are much more liberal than really necessary, so I can get away with using quite a bit less and still get good browning without burning.