Mixing different RAM sets is always such a thing. It can work in any case, but there may also be problems. (There are solutions in such cases too, but then you have to know about memory tuning in the BIOS, etc.)
Is this RAM listed in the QVL of the board? This is the list of RAM sets officially verified as compatible by the mainboard manufacturer; it can be found on the driver and support website or in the appendix of the manual of the respective board. If this set is listed there, this would increase the chances of success.
By default, both sets would run with the slower timings, but the difference between CL9 and CL10 is negligible anyway.
Bottom line, I would just try it and send it back if there are problems (sorry digitec).