How can I see a folder and a subfolder in the same window?

Yes I got it.. It's marlin file manager

If you want to install execute following commands in terminal:

sudo add-apt-repository ppa:marlin-devs/marlin-daily
sudo apt-get update
sudo apt-get install marlin

Hope it helps you..

This is one of the main reasons I always use List View:

It's not always ideal.

Some file managers have a multi-pane option. Nautilus used to but this was stripped back. There has been a bit of a struggle to get it back but I'm not sure how viable that is now.

I use Dolphin these days (I'm also a KDE user so that makes sense for me) which might be an option if you're doing this a lot. List view plus split screen FTW:

You can use Marlin, which is much less buggy than it was before, or Pantheon Files, which is forked from Marlin for the elementaryOS.

Installation instructions may be found on the following Q&A:

How can I colour code my files like on a Mac?