Central to the claim of Chalmers’ two dimensional semantics is the idea that there are two different ways of considering a possible world. We may consider it counter-factually in which case we hold the actual world fixed or we may consider as actual in which case the proposed possible world is thought of as being actual, with our world then a counter-factual world. Thus Twin Earth, considered as actual, is a world where water is not H20 but is rather XYZ. From their point of view it is a necessary truth that water=XYZ and so there would be no water here on Earth (considered counter-factually).
The problem with this line is that it assumes that we can tell a priori which possible worlds there are. Before we can consider a possible world as actual, we need to know whether it is possible or not. The two-dimensionalist simply asserts that some world is conceivable and then proposes to consider it as actual. But we have no non-question begging way of saying what is or is not really conceivable. Some claim that to be conceivable is simply to lack any contradiction in teh thing conceived. But just because there is no obvious contradiction in a proposed conceivable world does not mean that there is no contradiction in that coneption. Besides whihc, there are those who claim that contradictions are conceivable. Kripke famously proposed ‘non-normal’ worlds (sometimes called ‘impossible worlds’) these are worlds where contradiction are true or where standard laws of logic fail to apply (like the rule of necessitation). Are such worlds conceivable? Kripke seems to think so, others do not.
Until we resolve this issue two-dimensional semantics is hopelessly question begging. The only thing that we can consider as actual is, well, the actual world.