Can I use two different DDR4 RAM kits together?

You can usually mix them, but the system will generally need to run at the settings of the slower kit. Set the memory speed and timings manually to match the slower pair instead of relying entirely on automatic settings. For example, if one kit is rated for 3200 MT/s and the other for 3600 MT/s, you may need to run everything at 3200 with the looser timings.

The bigger potential issue is using four sticks instead of two. Filling all four slots puts more stress on the CPU's memory controller, so a setup that runs two sticks at 3600 may only be stable at 3200, 2933, or lower with four installed. Be prepared to reduce the speed one step at a time if the system is unstable or fails to boot.

It often works without any major trouble, but compatibility isn't guaranteed just because both kits are DDR4. Test the system thoroughly, keep the purchase within its return period, and reset or adjust the memory settings if it refuses to start. Mixing kits is generally fine if you're willing to accept a lower speed.