Memory is only guaranteed to work as a set.
That said, you can often get it to run at the lower speed and higher voltage of the two separate pairs.
What is the specific memory model that you have now? That would be important in my choice to add a pair or sell the current pair and buy the set of 4.
I presently have 16gb, 2 8gb modules
DD4 3000mz 64bit 288 pin
Though that I would add 2 additional 8gb modules (16gb)
Most of the chips seem to be 3200 mz
Aslso wondering if the (pc4 24000) means anything I should be concerned about.