For most people the puzzle bowls help with their dog's fast eating...but you've tried that. I googled it and came up with, of course the puzzle bowls, and a few other ideas, one with a treasure hunt. It's difficult for you, I bet, because of your other dog. Here are some links to some ideas:
http://www.reddit.com/r/dogs/comments/1ahmt6/lab_eating_food_too_fast_any_tips_on_encouraging/
http://www.wikihow.com/Stop-a-Dog-from-Eating-Too-Fast
Also check some of the comments on this link:
http://www.mypawsitivelypets.com/2014/02/5-tips-to-make-your-dog-eat-slower.html
Maybe you've already googled and checked out those links??