It sounds like @Shino888 wants to do it at runtime, in the game.
The engine supports it, because it supports split-screen multiplayer. One way to get to the solution would be to look at how split-screen multiplayer does it, and then attempt to replicate that for a single-player setup.
I think the general idea would be to actually create two viewports, and make them share the screen, though – not to “display two viewports in one viewport.” It may sound like a small thing, but I think the general setup will be easier and fit better with what’s there if you think of it as “two viewports.”