Calculating energy accumulated in toroidal coil
The two cores are effectively in parallel (like resistors). So, calculate the net reluctance based on this formula for each part of the core: -

\$R = \dfrac{l}{\mu\cdot A}\$ 

Where l is length of the magnetic path, A is cross sectional area and mu is permeability of material. 

Calculate the composite reluctance and use that figure noting that reluctance = \$\dfrac{mmf}{\Phi}\$ etc..

mmf is ampere turns and phi is flux.