So the situation is that Black can choose whether to kill W1, letting W2 live, so that B2 dies, or to kill W2, with W1 living and B1 dying. White can force Black to make this choice by filling in the liberties of B1 or B2.
If the position is left like this at the end of the game, then all the stones are alive, so it is a big seki.
For B cannot be captured. If Black captures W1 (resp. W2), that enables White to play a new stone that can not be captured where B2 (resp. B1) is now. B1 cannot be captured, and so it is alive; B2 cannot be captured, and so it is alive. (White can guarantee to capture either B1 or B2, but that is irrelevant, as in the standard example from which this position derives.)
Although it is not a matter for the rules, it is of some interest to consider the correct play here. If Black captures W1, so that B2 dies (or captures W2, so that B1 dies) it works out that Black is 9 points worse off than leaving the position as a seki, so Black should not do this. If White starts filling the liberties of B1 or B2, I think Black need not carry out a capture until White has reduced the liberties to 3; White has lost 12 points, compared with Black's carrying out the capture immediately, because of the liberty-filling moves, so White is 3 points worse off than leaving the position as a seki. So both players should leave the position as it is.
Let me repeat that the previous paragraph is a matter for the players to decide during the game, not a matter of rules. (The most interesting position so far as the rules are concerned appears when White has reduced the liberties of B1 or B2 to 4. This is somewhat subtler than the initial position, but it still works out as a seki, and White has no better move than to continue reducing the liberties to 3 -- White has already lost more than 3 points in captures while filling the liberties, so having gone this far it is correct for White to continue.