"Does it mean I have to earn 2btc before 24 hrs or I will lose?" Yes, this is right!
But you earn a % of each dividend, so the total deposited must be bigger than this. Let's say you deposit 1 BTC, and there is alreay 1 BTC invested, so you have a 50% of the total ( 2BTC). So, in order to win, 4 BTC must be deposited during next 24hours, because you earn 50% (50% of 4 BTC is 2 BTC).
so you will depend on new deposits to be able to have payments to older deposits?
yet you are in denial saying this is not a ponzi

But you can win or lose depending on your strategy.
For example, right now the first two investment have lose, because they didn't reach the target, so the rest of investments earn quite a lot and have a good chance to win.