Well butter and lotion are both moisturizers for your body. It's just that body butter is a lot thicker and more rich. You can use that if your skin is feeling really dry, or if you just want your skin to be crazy soft. I don't see why you'd need to use both at the same time.
I'd probably recommend body lotion during the day and maybe body butter before you go to bed, or in the evening, because it can take longer to sink in and it's heavier than lotion.