NSSplitView actually has a private method -(void)_setSubview:(NSView *)view isCollapsed:(BOOL)collapsed that does this. Those who would like to ignore all warnings against using private methods, behold:
- (void)toggleSubview:(NSView *)view {
    SEL selector = @selector(_setSubview:isCollapsed:);
    NSMethodSignature *signature = [NSSplitView instanceMethodSignatureForSelector:selector];
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
    invocation.target = self;
    invocation.selector = selector;
    [invocation setArgument:&view atIndex:2];
    BOOL arg = ![self isSubviewCollapsed:view];
    [invocation setArgument:&arg atIndex:3];
    [invocation invoke];
}
I implemented this as a category on NSSplitView. The only issue is that Xcode gives a warning about _setSubview:isCollapsed: being undeclared... I'm not really sure how to get around that.
El Capitan Update
I haven't written any code for OS X in ~2 years now so I haven't been able to verify this, but according to lemonmojo in the comments below, _setSubview:isCollapsed: was renamed in El Capitan to _setArrangedView:isCollapsed:.