Cassava is grown between rows of trees, you are correct. But it is the wrong thing to do, especially once trees are 2-3 years old and a proper root system is taking place. The correct procedure to remove established cassava is with a tractor and ripper. Worse than using a plough, and hence cutting all your nice growing roots. The only commercial thing recomended for rubber trees are pineapples, and stopping in the 3rd year. Year 4 onwards, nothing but your rubber trees on a rubber plantation.