When you have two windows snapped in Windows 10 , is there a way to Alt-Tab to both of them at the same time?

You can hit AltEsc to drop the full-size window to the bottom of the stack, revealing both of your tiled windows at once.

I'd investigate multiple desktops - then you could just switch to the desktop containing both your apps.

From Microsoft Support - Multiple desktops in Windows 10

Multiple desktops are great for keeping unrelated, ongoing projects organized, or for quickly switching desktops before a meeting. To create multiple desktops:

On the taskbar, select Task view > New desktop .

Open the apps you want to use on that desktop.

To switch between desktops, select Task view again.

You can then switch between desktops using CtrlWin← and CtrlWin→.

Virtual Desktops in many forms are a great way of dividing up your workflow. I've been using them since the late 90s on Mac [as a 3rd party add-on at that time] & couldn't live without them. It was eventually bought from that developer & added to OS X in 2007. Windows has had it natively since Win10 [2015]. It was first used on the Amiga in 1987 and unix X Windows, in 1989.

A focused, visible, and non-maximized window can be minimized quickly via Win + ↓ (Windows + Down Arrow).

Your most recently minimized window can be restored to its original state using Win + ↑ (Windows + Up Arrow).

A focused, visible, and non-maximized window can be maximized using Win + ↑. An already maximized window will experience no change.