I don't know how two aileron servos would affect the elevator, but having two servos is beneficial in a lot of ways. More power to the control surface, ability to mix in flaperons, helps to prevent or stop flutter issues, allows you to have shorter control rods = more responsive control surface.
As far as details on the install
here is a pic
http://www.henrylx.rchomepage.com/Pic/webIMG_3078.JPG
I use all 4 mil for my wings (only size I can get locally) so I have never found it necessary to reinforce to coro for the servo install nor have I found a need to screw the servo into the spar. (not that I am saying to do so would be wrong) I like to have real short control rods so I mount the servo about 4 inches from the control surface. I make the hole for the servo real tight so that the servo and the coro support each other to prevent and flexing then just poke a couple of holes thru the servo screw holes and run some zip ties.
Henry