Is it possible to mix rams with different latency
Yes, you can mix DIMMs with different latencies, although you cannot guarantee stability in all cases.
Some people advise buying four DIMMs as a single kit, not two "identical" pairs. Even when you buy two pairs with exactly the same part number, there's no guarantee the memory chips and sub-timings will be exactly the same. Mixing two kits with completely different part numbers just increases the risk.
Depending on how your motherboard interrogates the SPD chip on each DIMM, it may pick a set of timings which are marginally "too fast" for the slowest pair, on the assumption you've fitted four identical DIMMs.
Or it could run the DIMMs with the timings of the slowest pair, which will be a better solution.
The only way to find out is fit two (dissimilar) pairs and check the actual timings set by the BIOS. Then swap the pairs over in the mobo and check to see if the timings have changed. Choose the slower CL18 timings in preference to the faster CL16 settings, if your original pair are CL17.
A much better solution would be to ditch the old 2 x 8GB and fit 2 x 16GB or 2 x 32GB. Two DIMMs are usually more stable than four DIMMs made up from two unmatched pairs.