Can current transformers be cascaded?
You can connect the current transformer outputs in parallel, which will look something like the below. This is called a "summation CT" arrangement. The "A" ammeter represents your sensing circuit.



Industrially, we use a similar arrangement to implement frame earth fault protection on HV switchboards. Make sure you get the polarity of the connections right. If you get one of the connections the wrong way around, you will make a "subtraction CT" arrangement which has zero output if the two circuits have equal fault current - not what you want!

As for all CT circuits - ensure you never open-circuit the CT outputs, and ensure that your sensing circuit can withstand the maximum prospective fault current for the required time.



PS: Don't connect CT's in series. You end up with contradictions like the below. 

Contradictions in CT circuits are usually resolved with a 'bang'.