To play two games simultaneously on your OnePlus Device follow the following steps:
Step 1: Open your GAMES app by searching it from app drawer.
Step 2: Find the game which you want to play beside your main game.
Step 3: Press and hold on the selected game and click on remove. (it will remove that game from the GAMES app)
Step 4: Open your 1st Game and open the game space by sliding from the upper left or right corner.
Step 5: From the game menu click on edit, and find your 2nd game.
Step 6: Click on the plus icon on the upper right side of your game and then click on the DONE button.
Step 7: Your 2nd game is now added to the games menu, click on that app from the game menu. It will open as "picture in picture" mode. You can resize it and also can close and open it whenever you need it.
NOTE: In order to play 2 games simultaneously, both of the games must be in the same orientation. For example:
You can play PUBGM and Genshin Impact at the same time as they open at landscape orientation.
But you can't play PUBGM and Clash Royale at the same time cause Clash Royale is portrait orientation while PUBG is on landscap
