Need Dessert ideas for a cinco de mayo themed couples bridal shower for 50 guests.

I second the Mexican Wedding cookies idea! They are delicious and you could mound them up on decorative platters or put them in little baggies as favors.
Also, with the two rich cakes, how about a spicy fruit salad to cut the richness and offer a lighter choice? I had the most wonderful fruit salad with chili powder and lime juice on a recent trip to Mexico, freshly prepared in a cart alongside the road. Here is a recipe: http://vegetarianfoodrecipes.org/vegetarian-mexican-salad/spicy-mexican-fruit-salad. It would be best with melons, papaya, and mango.

Here is a cute idea for a candy buffet which is simple, fun and different
http://www.celebrationideasonline.com/candy-buffet.html

I also found a link to Cinco De Mayo themed party ideas with fun games, food, favors and ideas
http://www.celebrationideasonline.com/mexican-theme-party.html

choclolate churros or tamales, yum!

patty, the dishes you mentioned sound excellent. Do you know about Mexican Wedding Cookies? Those would be a natural.

Thank you for the suggestions. I am leaning toward a chocolate ancho chili cupcake with Choc buttercream w/coffee or the chocolate Patron liqueur and I might do the Tres Leches cake and incorporate coconut milk or coconut rum. I may try these out before the party and see what I think.

How about Fruit Enchiladas? You put pie filling in tortillas and roll like enchiladas and soak in butter and sugar. Can give you the recipe if you like.

I second tres leches cake. I love it, but don't see a recipe here. I did find a flan recipe that looks fantastic, and if you wanted, you could sub out the cardmaom (which I don't think is particularly mexican) for a bit of cinnamon and vanilla.

http://www.food52.com/recipes/9351_cardamom_coconut_flan

Tres Leches: You could leave out the gran marnier or sub with 1 T of vanilla--which is what I would do, I think

What about a Tres Leches cake?

What a cool idea! Here are Mexican desserts from Food52:
http://www.food52.com/recipe/Mexican