Hi,
I have been reading this with interest as I am thinking about installing some small Halogen lamps and was planning on running them on a lamplinc. It sounds like I want to stick with a 120V lamp to avoid a transformer if I want to dim them.

As far as cascading dimmers I would expect this to be an issue if they are solid state or triac driven. Triacs turn off at zero cross of the 60 hz power line and need to be "triggered" back on again each half cycle. I would expect there to be an issue with syncronization of the two triacs in series. If one truns on and the other is not on at the same time there might not be enough of a (minimum) load for for the first triac to remain on.
I do not have first hand experience with this but this might be one explanation for what you have experienced.

Regards,
Ela