As long as the kit you have stays in the same channel together, just look for another 2x8GB kit with the same (or better) speeds and sub-timings. Even if you did find the exact same kit as you currently have, there's no guarantee that both kits will run at their advertised speed together anyway (they're only guaranteed for the number of sticks the kit ships with).
Not sure how/if a single channel + dual channel setup would work. If you only want 24GB, maybe just add a 2x4GB kit?
Keep in mind that there's less actual RAM chip manufacturers than companies that sell sticks of RAM.