I got the RetroTink 2X Pro hooked up to the new Dell now too. It switches seamlessly between the inputs when the 7800 or the A8 is powered on. There is one quirk which is livable. When one is in color, the other is in grayscale. Pressing the input button cycles through to color - 2 presses for color after a device input change. No biggie, just make sure the RetroTink is accessible. I'll velcro mount on the back side of my Atari credenza with the top flush, and the button is just about .5 inch or so.
The picture is pretty good from the A8 using 800XL with chroma mod on monitor port (else it just outputs grayscale) using a monitor to s-video cable. The picture from the 7800 which is also AV modded (no modulator), and only outputs to CVBS, is acceptable too. I took pics but I had not set the aspect ratio first.
I set the SE2722H to Movie mode to get the best saturation, again it remembers.
Here are some pics of the A8 (with and without aspect correction):
Seems a tad too blue, so I may move the monitor to Cool mode, and adjust the saturation manually.
Here is a Rasta image in 4:3:
For the 7800, I took these pics (before setting 4:3 mode, sorry):