Alienware Aurora R5 - Ram Question

I purchased an Alienware Aurora R5 and the PC I purchased has a single 16gb 2Rx8 PC4 2133P Ram-module in slot 5 (XMM2) and I was wondering if I can purchase another 16gb 2Rx8 PC4 2133P module and stick it into slot 7 (XMM4) to get a total of 32gb ram. In the manual, it shows 4-8gb sticks to get a total of 32gb of ram. I would rather utilize the 16gb of ram I already have and not have to buy 4 sticks of 8gb ram. Can someone please let me know if this is possible?