I spliced my 2nd cig outlet into the existing one. They only charge my phone/run my gps so power draw is minimal. I've done it this way in my last 3 cars/10 years of driving without issue.
I wouldn't EVER take a 10 amp fuse and replace it with a 15amp fuse. It's a 10amp fuse for a reason.
I have also used "add-a-circuits" to the fuse panel on like 8 cars to add power for stuff. I'd go that route if I didn't splice in like I did.