How to install any app as a parallel app in OnePlus 6? OxygenOS in some OnePlus devices allows its users to create a clone of an existing app using a feature called "Parallel apps" under Settings app. The problem, however, is that only a handful of apps are listed as "eligible" to be run in parallel. As a result, the user has to resort to third-party apps to run an app of their choice in parallel. To make a case here, "Firefox for Android" cannot be installed as a parallel app!

I'm looking for a workaround for this superficial limit of only eligible apps to be run as parallel apps.

Note: I am using a OnePlus 6 running OxygenOS 10.3.2 (Android 10) with Magisk and Riru.
The solution is quite simple. Setup adb and issue this command:

adb shell pm install --user UID PKG


where:


UID: the uid of the "Parallel apps" user. To get the UID, you would have to create the user first. Find Parallel apps setting in your Settings app, enable any of the eligible apps (as that would create the user in the background), then disable that app you just enabled. 

Ordinarily, the UID might be 999, but OnePlus can change the UID in future. So to ensure the UID of Parallel apps user, run the command:

adb shell pm list users       
adb shell cmd user list      # alternative command


Example output:


Users:
    UserInfo{0:Owner :13} running
    UserInfo{999:Parallel Apps:4000030} running


The numeric string before ":Parallel Apps" is the UID of that user.
PKG: the package/app you are trying to install


Lo and behold, you have an app running under Parallel Apps. 

Tip: you can install any app, regardless that the app is installed under the primary user or not. You can also install a third-party client for Play Store or an altogether different market app to install/remove apps from Parallel apps user conveniently. See this XDA thread for more details.

Issues:


The "Parallel apps" setting under Settings app would not list these apps.
If your parallel app is indeed a clone/duplicate of an app running under primary user, than removing the primary user's app would automatically remove your sideloaded parallel app too. So, if Firefox is installed as both primary and parallel app and you happen to remove primary Firefox app, the other one would be nuked too.
User beeshyams mentioned that TWRP backup/restore functionality breaks if "Parallel Apps" user exists.