Working on multiple research projects as an undergraduate with different professors
If he is just concerned that you will not have enough time to do both, it's up to you to prove that you can handle it. 

But a general feature of academia is the presence of a lot of difficult and fragile personalities. It's not impossible that Prof. A took your choice personally, especially if there is any behind-the-scenes tensions between Prof. A and Prof. B. It's worth thinking about these things, but not too much. If Prof. A is going to be professional about it, there should be no further repercussions in your relationship with him. 

To elaborate in response to your more specific questions... 


  Was I wrong in not going to Prof A for advisement about this new opportunity, considering all he has done for me? 


No. 

I can't say whether or not you communicated with Prof. A in the most optimal way. Maybe things would have gone more smoothly if you said something sooner. But on the other hand, maybe you didn't need to tell Prof. A anything at all. There is no set rule or etiquette, it just depends on what your relationship with him is like.


  Or was I correct that I have the right to go and find other
  opportunities if I wish?


Yes.

You have every right to pursue whatever opportunity you think will be best for your education and professional development. Do your best to maintain good relations with the people you work with, but don't let that stop you from doing what you feel you need to do.