[EricVanWyk]

DISCLAIMER: There is an 85% chance that I am completely wrong.

Servos are nominally position controlled, but you want them to add their force together. To me, that indicates that you want to make them force controlled.

If you add a bit of series elasticity (a rotary spring), you might be able to add enough slop so that they aren't fighting each other so stiffly.

I'd prefer if a MechE chimed in here. It just seems analagous to a few things I do as a SparkE. For example, lets say that I need to use several linear regulators in parallel. These are nominally ideal voltage outputs - very stiff. However, they will each have a slightly different idea of what 5V is, so one of them will take over. One solution is to add small resistors afterwards to ensure load balancing. This takes the very stiff controls and give them a more forgiving slope.