Hey there @da.st! Unfortunately rendering two viewports real time at once is currently handled only inside of the split screen portion of the engine or in third party plugins. If you’re using blueprints this will likely be your limitation altogether. Although the engine is capable, you’d just have to work from source (C++) to accomplish it.
Some users are reporting they can pull this off with the Composure plugin, while it’s not it’s intended use, it may be cheaper than using render targets (unconfirmed), and less hacky than adjusting the split screen.