It all boils down to your motherboard implementation.
Ram can be implemented several ways, referred to as "channels".
Single channel refers to ram accessed one stick at a time. If you have 2 ram sticks, the CPU will start at stick one and fill that before putting anything in stick two.
Dual channel refers to accessing memory 2 sticks at a time, to get by having to wait on ram. The first datum will be stored in stick 1, the second in stick 2, then back to stick one, etc. This used to be called "Memory Interleaving".
Quad channel is the same idea, but using 4 sticks in the "round Robin"
The latest intel board released for the next CPU coming supports "Hex channel"
Each of these improves system performance over the previous method. Most systems these days support dual channel, hence having 2 ram sticks installed. You must use matched ram sticks as a pair but each pair may not need to be the same... Again, consult your manual to see what is supported.
Adding a 2GB pair should work, I just didn't think of that option... I usually look to optimize my slots
If I upgrade this motherboard I'll probably add a 32GB pair and move my 16GB pair to the second bank, for example.