Two genetically modified dogs, one good the other bad

Starts with a dog being found by a hiker. Can understand all spoken English but hides it's abilities. Being followed by it's "evil twin", the other extreme of the experiment, that wants to kill. Read in the 1990s?

Book found: Watchers by Dean Koontz

Sounds similar to Watchers by Dean Koontz, which I read in the late 80s. I looked up the plot on Wikipedia, and I'm sure it's your book:

https://en.wikipedia.org/wiki/Watchers_(novel)

Thank you so much Moomin Mama! I have been hunting this book down for years, and you have found it for me in just hours. Now I just have to get hold of a copy........
Thanks again and best wishes,
Nicola

Glad to help. I remember enjoying it and I must give it a re-read someday.