C++ DC on top of another DC

I started to notice that when I write new lines of code, it starts to slow down, only speed and visual updates became slower but I want to understand how to connect DC’s and their displays, I tried to do it by just adding another function with While(1) which also calls everything new but there is blinking as one DC tries to overlap another DC but how to make one DC stay and at the same time the new one was on top of it.

In short, how to connect the first DC with the second DC to be on the 2nd layer on the display.