Is it possible to have more than one camera at the same time? Like the "real-time" multiple camera shots from 24.

Sure, define different camera with

view cam01
{
.....
.....
flags = visible;
}

view cam02
{
.....
.....
flags = visible;
}

I havent tested this and I done it in notepad. But you will get the idea:
It will place 4 views in each corner of the screen, works with any resolutions.

Code:

---

view camera1{flags = visible;}
view camera2{flags = visible;}
view camera3{flags = visible;}
view camera4{flags = visible;}

starter initialize_cameras()
{
	// Camera 1
	camera1.pos_x = 0;
	camera1.pos_y = 0;
	camera1.pos_size_x = screen_size.x / 2;
	camera1.pos_size_y = screen_size.y / 2;
	// Camera 2
	camera2.pos_x = screen_size.x / 2;
	camera2.pos_y = 0;
	camera2.pos_size_x = screen_size.x / 2;
	camera2.pos_size_y = screen_size.y / 2;
	// Camera 3
	camera3.pos_x = 0;
	camera3.pos_y = screen_size.y / 2;
	camera3.pos_size_x = screen_size.x / 2;
	camera3.pos_size_y = screen_size.y / 2;
	// Camera 4
	camera4.pos_x = screen_size.x / 2;
	camera4.pos_y = screen_size.y / 2;
	camera4.pos_size_x = screen_size.x / 2;
	camera4.pos_size_y = screen_size.y / 2;
}

---

replace this:
Code:

---

starter initialize_cameras()

---

with:
Code:

---

initialize_cameras_startup()

---

this is the new syntax for starter functions

I thought that the "_startup" thing was only in Lite-C. I use Lite-C, so thats why I thought startup was still a replacement for function (I never used to use startup functions).

And can the multiple cameras have different layers? Can they be overlapped?

I'm not sure, but perhaps you can place them on top of each other and make one 50% transparent.