I asked Dr SHimek about it & here is the response I got.
Hi Craig,
What will go on is not so much chemical warfare as "chemical signalling." Each of these animals produces a "chemical signature," a waterborne odor, characteristic of its kind. When this odor is sensed by another anemone, that animal may become "aggressive." It will extend out its tentacles to extra long lengths, and it may bend or move upstream - toward the other anemone - with the intention of fighting with it. There is no "thought" to this, anemones, of course, do not think, but it is all the result of a series of reflex behaviors.
So, the real problem of keeping individuals of two or more species of anemones can be solved by eliminating their particular chemical exudates, which will keep the aggressive responses to a minimum. This can be done by filtering the water or maybe by proper positioning of the animals. They can coexist for a long time. A problem will occur, however, if a change happens and one or the other of them senses the "invader."
Aggressive anemones, in nature, fight until either one dies or moves away. In an aquarium, the loser really can't move anywhere, and the winner may pursue it to kill it. Damage to both animals can occur, and often both animals will die.
So... The bottom line, in a large tank it is possible to keep two or even more different anemones, but there is a potential risk involved. One needs careful placement, and careful water "maintenance" to ensure that no problem erupts.