Logisim: "Oscillation apparent"
Many kinds of latching circuits are prone to starting up in an ill-defined state.  In your nor-based RS latch, if on powerup both inputs are low and the gates are assumed to output low, then all the inputs to both gates will be low, so the gates will output high.  Then since each gate will have an input high, both will output low, etc.

A nice approach to the problem, if you're allowed to use it, would be to have one of the NOR gates of each latch take three inputs, and wire the extra input from both of them to an "asynchronous reset" signal which will be pulsed high on simulation start-up.  Otherwise, it will be necessary to ensure that both of your latches get clocked soon enough after simulation start that the simulator won't squawk at the oscillation.