Up for sale are some Ergotron dual monitor arms. I'm not sure of the exact model number but I believe they are similar to these ones:
I had a 24" monitor on these with no problem.
New these are $300+ dollars, but I'll sell these for
$50 as I'm clearing out a room and I'm missing the caps that cover the holes for the arms on the base (just a cosmetic thing)
PM if interested. For pickup in the Kensington area.