I'm also a bit confused by these two apparently contradictory statements you made:
Leverage of 2:1 means that your onsite investment can "control" a total investment twice its size. In other words, your offsite can be up to the size of your onsite (10 BTC in your example).
and:
Assuming your onsite is 10 BTC, your offsite is 20 BTC
If 2:1 means offsite amount is same as onsite amount, and 2:1 is the limit, how could he have 20 offsite and only 10 onsite?
2:1 is indeed the limit when adjusting your investment (although you won't be margin called if you later exceed it). I stuck with Johnson2239's example calculation anyway to show that it was off.