2 notes at one time

It could be a very nice feature, but I'm not familiar with a plugin that does that in Joplin's desktop version.

One thing you could do, is use Joplin's vscode extension. It gives you seamless access to Joplin's database, from within VS Code. Since VS Code supports a split view of 2 editors (or even more), you may open and edit multiple notes in parallel.