Matchoxes will definately work for dual elev servos but in almost all cases, its not needed if your using a computer radio.
You Mix the elev to the channel your second elev servo is in. Now you have subtrim and epa on both sides independant of eachother.

The real use for a matchbox is when you have two or more servos on a single surface. This allows you to subtrim each servo and epa each servo independant of eachother and have the whole thing plugged into one channel.
You need this even if the geometry is perfect because the output splines on servos are not always the same. This prevents the servo arms from matching eachother.
Think of it as a multi channel, individual servo adjuster, Y harness.