It wouldn't be 'harmful' to do both but it will change the taste of the tomatoes and it sure isn't necessary to do. The instructions make it clear it is either bottled lemon juice OR citric acid so yes you would pick one or the other.

"add two tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid per quart of tomatoes. For pints, use one tablespoon bottled lemon juice or 1/4 teaspoon citric acid."

Can I ask why you added both? What source/set of instructions did you use? How and how long did you process them since the processing times are different for quarts and pints?

Dave

Thanks Dave. I did see that in my Ball Canning book but was going on advice from a friend and gave it a try. According to the book I canned the quarts for 45 minutes in the Boiling Water Canner. I'm new to the canning world and wasn't sure how it would affect the tomatoes I canned the other night. In the future I will just use one. Thank you for taking the time to answer. :)