Custom segue, but leave the previous scene showing - objective-c

Question

Imagine a custom segue ...
-(void)perform
{
UIView *sv = ((UIViewController *)self.sourceViewController).view;
UIView *dv = ((UIViewController *)self.destinationViewController).view;
UIWindow *window = [[[UIApplication sharedApplication] delegate] window];
[window insertSubview:dv aboveSubview:sv];
[dv coverFromRight:0 then:^
{
[self.sourceViewController
presentViewController:self.destinationViewController
animated:NO completion:nil];
}];
}
Which in fact, only PARTIALLY (!) covers the "underneath, previous" scene,
and in fact DOES NOT call "presentViewController", so, the "underneath, previous" scene in fact keeps operating normally.
-(void)perform
{
UIView *sv = ((UIViewController *)self.sourceViewController).view;
UIView *dv = ((UIViewController *)self.destinationViewController).view;
UIWindow *window = [[[UIApplication sharedApplication] delegate] window];
[window insertSubview:dv aboveSubview:sv];
[dv coverButOnlyHalfWay:0 then:^
{
}];
}
Essentially, is this possible?
In fact, I've found from experiment the above works (!!). BUT when you come to the custom unwind segue, it does not work: everything crashes. (Perhaps as you'd expect.)
What's the situation? is there a way to make a custom segue, which, covers only say half the "original, underneath" scene and leaves that scene running?
(I appreciate you can just implement this using a container view, but it's not as clean as a whole segue scene.)

Answer 1

Why use a segue? You can just add your view as a subview and position it correct using CGRectMake, this would be much easier.
// Size Your View with X, Y coordinates
[viewController.view setFrame:CGRectMake(0, 0, 320, 192)];
[self.view addSubview:viewController.view];
[viewController didMoveToParentViewController:self];
[self addChildViewController:viewController];