It's always best to go with a kit.
If you don't go with a kit...your chances of them working together are generally less.
No one can predict whether the two will work together.
You have to try them to find out.
Most of the time when I mix memory like this...it works. Sometimes....it doesn't.
That doesn't matter. The real worry is whether they are actually the same. GSkill is not a manufacturer of RAM chips. One module may have chips from Samsung and another from Micron. With slightly different timings. It all depends on who was cheapest at the time when an order for components was made.
The RAM will have the same model number. But the subtle differences could ruin all XMP profiles. Requiring them to run at a slower common timing. Sometimes they may not work together at all. Sometimes you'll be able to manually OC them for similar timings and speed to the original XMP profiles.
Then again they could be identical and work exactly as you'd expect from a factory paired set.
It's a crapshoot. You never know until you try. Certainly your odds are much higher of an exact match by buying the same model number.