What is your motherboard?
Your motherboard manual will tell you which two slots should be populated if you have 4 and are installing only two sticks.
Similarly, where to install only a single stick.
Not all motherboards can work with a single stick in any slot.
Past all that,
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
If you can get into the bios with only the Kingston stick, see if you can increase the ram voltage from the default 1.5v up to 1.6v.
Test that out and then install your second stick.
If that fails,
My suggestion is to return the g.skil stick and buy a 2 x 4gb kit that matches your Kingston specs.
You will have 8gb that you wanted.
Then try to install the old 4gb.
If it works, fine, you now have 12gb.