With and without branching

Test1.m

@implementation Test1

- (id)init {
    self = [super init];
    if (self) {
        [self addObserver:self forKeyPath:@"property1" options:0 context:NULL];
        [self addObserver:self forKeyPath:@"property2" options:0 context:NULL];
    }
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:@"property1"]) {
        // Do something.
    } else if ([keyPath isEqualToString:@"property2"]) {
        // Do something.
    }
}

@end

Test2.m

@implementation Test2

- (id)init {
    self = [super init];
    if (self) {
        [self addObserver:self forKeyPath:@"property1" options:0 context:@selector(property1DidChange:)];
        [self addObserver:self forKeyPath:@"property2" options:0 context:@selector(property2DidChange:)];
    }
    return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    [self performSelector:(SEL)context withObject:change];
}

- (void)property1DidChange:(NSObject *)change {
    // Do something.
}

- (void)property2DidChange:(NSObject *)change {
    // Do something.
}

@end