haven't used a 6exa, but i owned a 6xas for quite a while. it IS possible to do it, although it is a very poor method as your trim only works on one and if you happen to hit the wrong switch while in flight, bye bye elevator control. in other words, it is much easier to buy a reverser for this, that is what I did for my dual elevator servo ucd46. I think futaba itself puts it best
"
Also, the elevator dual rate switch MUST be on at all times or the mix is turned off and your model will, very likely, crash"
but... if you want to do it, here are the instructions for the 6xas, should be similar.
http://www.futabarc.com/faq/faq-6xsuper-q473.html