Two state legislatures are weighing bills that would allow lawmakers to override certain state court decisions, drawing concerns about separation of powers.

Proposed bills in Florida and Washington would permit legislatures in those states to vote to “reject the determination of the court,” as the bill in Washington puts it, following rulings that declare a legislative act unconstitutional. In Florida, the proposal would require a two-thirds vote of each house within five years of a court ruling. In Washington, a simple majority...