On the other hand a second washer could be enough to get the element up out of the combustion chamber enough to slow combustion and reduce the engines efficency and power. Certain engines require a long plug for a reason.
It also introduces another "weak link" so to speak that could be an air leak.

Just use one washer and if you feel the need to change the compression do it with the shims.

And one other thing, you do know that glow plug washers are directional, right? Look closely at a new washer, you'll see that it's "bowl" shaped. The bottom of the bowl goes towards the glow plug and the rim goes towards the engine head.