@kanakro Absolutely! It may not be too obvious, but it's actually being done in @bendmorris 's example! If you look closely, inside of the UIScene update the gameScene is being manually updated. Like you have found, only the last added scene will be operational.
The trick here is to pass uiScene a reference to the game scene, and to make sure uiScene is the last scene pushed on the stack. Then both are able to render.
There is one caveat though, as you found, if you pushed two scenes on to the stack at the same time only the last one will be rendered; but if you had pushed on your uiScene after the gameScene was running for a while then you'd see your rendered square, the scene logic wouldn't update anymore.