From looking at the HP-really-Ergotron desk stand, and looking it up on Ergotron's website, it says this:
Weight Capacity: 7–25 lbs (3.2–11.4 kg) for single monitor. If a second accessory monitor arm is added to the base, maximum recommended individual monitor weight is reduced to 20 lbs (9.1 kg). Monitor depth may affect weight capacity. Contact Ergotron for details
HP Stand (dual made single)