Well, assuming you both in fact have oral herpes (cold sores), then it gives you each significant protection against transmitting it genitally via oral sex. Just don't do it when you have an outbreak, that also protects you...
I was under the impression that canker sores are not HSV1 that only cold sores are. I also thought you could only get cold sores on the outside of the mouth. I just tested positive for HSV1 and plan on posting my experience with some questions, but I hope this helps. - if it's right that is