When there are 2 available butterflies of the same species the "fuse" button will be lit up when you select the butterfly, and you will be able to push this. A circle will appear around the correct butterfly in your forest and you should select it and they will fly together and merge. To fully fuse and level up a butterfly you will need to hatch the same species 5 times, fuse them together, and feed them the required amount of pollen until they glow blue! Once they have been fully fused you will get the maximum honeydew harvest amount, and be on your way to completing an entire set.