My smartphone has ability to use two sim card or one sim card plus sdcard. Generally, I'm using second option (sim+sdcard), sdcard as adopted storage. I want to use second sim card temporarily. But if I will unmount sdcard and pull it out will I be able to use it later without problems?
It works as expected. I successfully unmount adopted storage.
First I moved all apps to internal memory. Then I unmount adopted storage from SD card options.
System starts syncing data that was not written yet and after minute there was intent notification "You can remove sdcard safely". So I removed card. Nothing wrong was happening. Of course, wallpaper disappears but it was predictable.
Another side effect was that I can't save screenshot so I made these photos by another phone.
After some time I inserted same SD card to smartphone and Android mount it automatically. Wallpaper appears again and all my files from SD card was in the same place as before.
Technically adopted storage is separate file system but mounted in a file system, in the specified directory. Not every app can be moveable to adopted storage. It behaves like normal SD card with FAT/exFAT file system but Android merge view of internal memory directory in one "file system". Another difference is that adopted storage is encrypted. Android mounts that encrypted block device, decrypt it and mount in /mnt/expand/. No system data are moved to adopted storage, only apps that developer implement an option to move app data to adopted storage.
Test was made on Android 7.1.1. Smartphone model: Motorola G5S Plus (XT1805)
Once you've adopted your external SD card as internal storage, you no longer can simply remove it without causing massive issues (apps crashing and the likes) – after all, the card is acting as internal storage then, covering everything that goes to
/data/data (i.e. all the apps' data, configurations etc.).
The only way to take it out without causing issues is to convert it back to "portable storage". Which is technical possible – as long as there's sufficient internal storage available. If there is, I'd wonder why you've adopted the card – so that probably won't be done by just pressing a single button: you'd first need to make enough space, i.e. remove data stored in "adoptable storage" so the remains fit into real internal storage again.
TL;DR: No, you cannot simply unmount and take the card out to free the shared slot.