There is no requirement of 24GB of RAM. The Titan X has 12GB of GDDR5 , 2 of them would be 24GB of GDDR5, but in SLI only 12GB is usable (until DX12 comes out, which may allow for memory to stack).
You will be fine with 16GB of DDR4. What i think you were getting confused about is the amount of GDDR5 the Titan X has, and thinking you needed the same amount (or more) of System RAM.