Sullivan make an "elevator splitter".
http://www.sullivanproducts.com/ContSysAccMainFrame.htm

If the servo is mounted with the arm motion happening in the same plane as the "dumbbell" (see the picture on Sullivan's site) then the dumbell will rotate slightly and the you will end up with differential movement between the push rods.

If you setup things up so that the servo arm and dumbbell are perpendicular then this is not a problem. If it can't be done this way, I wouldn't recommend it (having tried myself).

That may not be clear.... how about this: you don't want the little dumbell moving side to side as the servo arm moves because it (the dumbell) will rotate slightly.

Here's the luxury solution: http://www.centralhobbies.com/contro.../Bellcrnk.html

.... soldering two pushrods together will work as well.