What is the ideal travel for dual crown fork?
Given the frame is designed for 100mm fork, its unlikely its strong enough to handle riding that needs a 160+mm travel fork, and frame failure is possible. Its not the dual crown that is the problem - its riding a frame designed for 100 travel (typically XC) in a way that needs 200mm travel (typically extreme down hill). 

Changing the fork travel changes the frame geometry and compromises handling. The original fork has 100mm, the bike is not designed for higher travel forks, anything more that about 120mm would starting to affect the handling. Above about 140mm, the bikes geometry would be all out of kilter, and it would be a completely different feel to what it is now. 

For the same price, modern single crown forks in the 120-140mm travel are as good as any dual crown for enduro riding.  The original fork being a XCM100 also means its an entry level frame. I doubt the frame would benefit significantly from a decent dual crown fork over a decent single crown which would be nearly as stiff and lighter weight.    

Adding all this together, if you really need more than 120mm travel, you would be better to buy a bike built for it. If you only need 120mm travel, a single crown fork will do the job best.