You only need one Sonoff, all two-way and three-way circuits are just like points on a train track that switch the route and make or break the circuit with the power fed in at one point. the wiring at the light fitting will be the same whatever number of ways it is, the extra is just more wiring looping from switch to switch.
In this two-way diagram, you’d remove 2 & 3 and attach them to gpio 14 and gnd on the Sonoff. Remove 1 & 8 and attach to the output of the Sonoff. Then attach a wire from the block with 4 & 5 and another from 6 & 7 to the input on the Sonoff.
Job done 
