It depends on the motherboard.
Some have 4 slots and 2 are for one type of memory and 2 are for a different type. You can use one pair or the other but not both.
Other motherboards that have 4 slots might take dual channel memory which means that you upgrade in pairs.
It depends on your motherboard. I would suggest that as yours is working ok you have it right.
If in doubt look at Crucial and either identify your computer for them or let it scan your computer, it will tell you what you have and what you can take. click here