Using more than one design in a room leaves the eye with a lot more to gobble up, however it can be difficult to know the perfect level for balance. Although a room's more interesting and unique with two flavours to take in, too busy and your eyes can get distracted and pulled different ways at once! The easiest way to make sure that you can find success and balance between your spa and heavy metal death themes is to find some common ground and build up around that. Using the same colours throughout can be a brilliant way to make themes, although different, seem like they're flowing straight from the same river. Whacking a focal piece, that fits both design styles, such as a much-loved sofa, right in the middle of the room so that both designs can always relate to something right in front of you is great for reducing clash and increasing flash.
Just because you're going for two different styles doesn't mean you can forget all that fluff about lines and shapes and their adventures with each other, it's just as important, if not more important, to explore the lines the eyes follow when looking at different furniture styles and make sure they're still pals and flowing together, otherwise your eyes won't know where to look!
Never split different sections of the room for a specific style, you should want to mix them all equally so the difference isn't too big and it's easier for the eyes to lap up. When picking furniture of different styles always aim for them to be roughly the same height and take up the same space; it can feel odd if even one piece of furniture is taller than the rest- but a mix of different heights is a recipe for eyebrow raising!