Here's three methods I've tried. By far, the extended pushrod method is the best but it requires access to the rear fuse so it's not easy in composite fuses.

1. Extended pushrod: http://home.no.net/aerowold/myplanes_saphir.htm
(scroll down, it's picture 9 and 11)
the pushrod is a simple balsa rod with dual ends, the neat thing is that the balsa rod extends into a hole in a former behind the split. I got this tip from Sigurdur Kristjansson, I believe it originates from Dean Pappas pattern column.

2. Two pushrods joined at the servo. They are supported by two formers on their way inside the fuse. Works fine but for big throws (3D) you need to mount the servo sideways since the servo will push one side further than the other at extreme throws when mounted upright. No problem for F3A though.
Photos:
- http://www.flickr.com/photos/aerowol...7594386135264/
- http://www.flickr.com/photos/aerowold/302949106/

3. MK Dual BB coupler.
The most common installation, see photos here: http://www.flickr.com/photos/aerowold/tags/skystar/