Для действия кнопки «1» ...
//For opening the second nib on first button click
NSWindowController * second=[[NSWindowController alloc] initWithWindowNibName:@"secondNIB"];
[second showWindow:self];
//For closing the first nib, optional
//firstWindow is NSWindow
[firstWindow orderOut:nil];
Для действия кнопки «2» ...
//For opening the first nib on second button click
NSWindowController * first=[[NSWindowController alloc] initWithWindowNibName:@"firstNIB"];
[first showWindow:self];
//For closing the second nib, optional
//secondWindow is NSWindow
[secondWindow orderOut:nil];