Ergotron – LX Vertical Stacking Dual Monitor Arm, VESA Desk Mount – for 2 Monitors Up to 40 Inches, 7 to 22 lbs Each – Tall Pole, Polished Aluminum

I have a 24" stacked on my 49". I pretty much only use it for screen sharing or parking messaging apps or spotify as my neck really does not like it. I use the ergotron LX vertical stack. To get the top monitor to lean forward more than a nominal amount you need to clip a tab off it but otherwise is great.