As long as you don't violate box fills or other restrictions, there's no reason you can't use a double switch (rocker or toggle).

If the switches are listed and rated for the use, it is strictly an appearance issue.

Thanks for the feedback! This photo shows how I don't have room to expand into a 2 gang box without intruding into what will be the new backsplash space.

Maybe I could expand that 1 gang box outlet that's over the countertop into a 2 gang box, put a normal switch there for the undercabinet LEDs, but wire it not off the outlet beside it, but off the existing light rotary-dimmer/switch that's 18 inches to its left?

Here is a link that might be useful:

I have a triple rocker mounted about 12 inches above a single rocker in the bathroom. The single is for the main lights and the triple is heat, fan, light.

Also have a double rocker in a double box with a single rocker. The double is overhead kitchen lights and the single is wired outlets in the dining area. Very easy to tell which goes with what and does not look odd at all.

Don't get the cheapest, they do look cheap. Spend 15 to 20 bucks.

I would totally stack them. They'll look modern and snappy, up-to-date.

And yeah, don't cheap out.
(Unless you're doing an entire house, or an entire neighborhood, I've never understood the appeal of rounding down on that sort of "hardware"--it's usu. not just a "looks" thing, it's functionality. And while the percentage difference can be high, it's really cheap compared to the cost of the backsplash itself.)