I get the feeling this Jerusalem Post article is slightly exaggerating the importance of this. I have no idea of supporting a two-state solution is even a shift in AIPAC's policies. And I can't tell if AIPAC is making lobbying for a two-state solution specifically a central program, or if it is just part of the overall agenda it is forwarding to Congress.
Even still, it is a good thing. The more Congresspersons on the record supporting a two-state solution, the better off we all are. And the more unified the pro-Israel community is on the point, the more we can dissipate the notion that being "pro-Israel" means being implacably opposed to Palestinian rights.