In a small pot, mix the guava paste with the water and bring to a boil over medium-high heat. Reduce heat to medium and let cook for approximately 5 minutes, stirring occasionally. The paste will be partially melted. Remove from heat and stir in the lemon juice. Let cool.
In a electric mixer, whip the cream cheese or mascarpone a bit at low speed. Add the partially melted guava paste and beat until well combined. Add the guava shells and mix well by hand. Reserve.
Grease with butter or non-stick butter flavor cooking spray a 9 × 13 inch (about 33 x 23 cm) glass baking dish. Layer 6 bread slices. Cover with the whipped softened cream cheese/marcarpone-guava mixture.
Cover with remaining bread, pressing down gently to seal the edges. In a separate bowl, beat the eggs, milk, coconut milk, condensed milk, spices, salt, and vanilla extract until well blended, and pour on top of the bread. Cover and refrigerate overnight or at least for 4 hours.
Preheat oven at 375 º F (190º C) and bake the Romeo and Juliet French Toast Casserole for 45-50 minutes or until set. Serve Romeo and Juliet French Toast Casserole warm with honey or sprinkle either a mixture of cinnamon-sugar or powdered sugar on top. Enjoy!