Try setting setUserInteractionEnabled:YES
before adding gesture recognizer.
[imageview1 setUserInteractionEnabled:YES]
[imageview2 setUserInteractionEnabled:YES]
[imageview1 addGestureRecognizer:singleTap];
[imageview2 addGestureRecognizer:singleTap1];
Update:
After the comment you have made I suggest you bring your views to the top before detecting the tap event. Because parent imageView is above and catches these taps.
[yourparentview bringSubviewToFront:imageview1];
[yourparentview bringSubviewToFront:imageview2];