I’m almost certain it’s additive. Because accord to my credit history, it say I have 19-20 years of credit history, and I’m only 26. I do have several cards. So it only make sense that it’s additive. Remember, new cards/loan only hurt your credit the first 18 months, after that, it becomes helpful to your credit. So if you must open new cards, I suggest open all you would open in one shot, so 18 months from now, there would be no negative marks on your record. If you stagger them, then that period would be much longer.
