I am writing a solver for a system of 12 coupled PDEs. Since some of my equations are advection-dominated, I am implementing an FCT-stabilization algorihm, which needs the solutions at time steps n and n-1 in order to calculate the flux corrections and solve the system at time step n+1.

I checked some source files and found out that the solution at time step n is stored in

I don't know for sure, but I think you would need to store it by yourself. You can examine the adaptive timestepping code for a possible way to do that.

The number of previous timesteps that are stored depends on the time stepping method. If you use bdf then there is the keyword "bdf order" that defines the order, and this is also the number of previous timesteps required: N:th order scheme requires N previous timesteps.

Currently this may be married with the timestepping strategy. So if you do you require 2 previous steps, but use a 1st order scheme then you may need to do it yourself. The order of the solver is found in "Solver % Order".

It would probably be a rather small modification to the library to allow number of steps to be stored be larger than order of timestepping scheme but to my knowledge it has not been done.