can i mix ram of two different companies??



yes you can use both at a time, but it is recommended to have exact same model to avoid unwanted problems.
 


unwanted problems like??
 
For the best result and problem free computing it is advisable to purchase all the RAM you are going to install in a bundle pack. Even using the same modules from just one manufacturer can cause problems if they are not bought as a bundle. Speed variations between the sticks can cause erratic behavior.
 


Failures to boot, random crashes, not using / reading all the RAM (incorrect # of GB's) -- to name a few.
I agree with Tradesman1.
To emphasize: I rebuilt my wife's PC several years ago. Mobo changed from a 3 DIMM board to a 4DIMM. I had 3 matched sticks of RAM from Crucial. I shopped for a 4th one and when I found it I talked directly to Crucial Tech to compare the properties; they assured me the new Crucial RAM would work with the old Crucial RAM.
Once installed it did in fact "work" but I was NEVER able to set up dual channel memory so it operated as single channel. Not too many months later I replaced that RAM with matched set of Corsair and Dual-channel fired right up.