Interesting problem you've got there, with sun on one side and shade on the other. No, you don't have to match both sides of the foundation garden, but it is important that you make the differences look intentional. I'd suggest a more informal design (soft, flowing lines rather than geometric, and use of a range of plant heights and shapes). If you're having trouble growing things in the sun, check the soil and make sure that's good before spending money on plants. For sunny sites, there is a huge range of plants you can grow. On the shady side, you still have a lot of choices, but it will be more difficult to have color. I would suggest leafing through some plant catalogs, garden magazines and books to find plants you like. Make sure they grow in your area, and then look to link them with some visual characteristic such as shared color or foliage texture. That will help draw the diverse plant palette together. Good luck!