Likely you can, but there's a reason dual memory (or quad) kits exist: they've been tested at the factory to work together with no errors. Like CPUs, memory from one day to another in fabrication (silicon specifically) can have microscopic variations that may or may not cause one module to play nice with the other.
So essentially when you buy memory individually and not in a kit, you are rolling the dice. I've noticed this decreased tolerance level of using non-kit memory modules over my 20 years of PC building. For example, I recently upgraded an old Core 2 Duo laptop with a different second 4GB DDR2 memory module to pair with the 2GB that it came with for 6GB. I tried upgrading my Sandy Bridge HP Pavilion laptop with a simple second...