I started out using just the Ceramidin liquid a few weeks ago- both alone and under my regular moisturizer. Somedays your skin just doesn't need as much moisture, and I think you can just use the liquid on those days. It works that well! I just purchased the Ceramidin cream yesterday to see how it works over the liquid and it's incredibly hydrating and firming. Both together will give you a gorgeous dewy glow and I find that my skin looks very taught and firm when using both the liquid and cream together. For what these two products offer for skin repair, they are incredibly affordable. These two products can replace a lot of clutter in your cabinet. They really work.
The theory is that the Liquid product hydrates the skin, and the Cream product locks in that moisture and protects the skin from losing it. It's a bit of a gimmick, IMO, but I do love the products. However it's not necessary to have both. The liquid is a light moisturizer, so if that's all you need you might like it by itself. If you have very dry skin, it won't be enough for you and you'll need the cream (or some other heavy moisturizer) on top. The cream is fantastic for dry skin. If you only bought the cream, you might want to layer it over something light like a light lotion or a hydrating spray toner, but it could be something from the drugstore.
It's not imperative to have both, but if you have the budget to buy both of the ceramidin products, they do work well together.