No difference really, even if you do a 3 year degree you will probably do Part IIs once you graduate while you're working. Go for the double if you don't wanna leave uni so early, or you have a double degree you're really passionate about, or you want an extra year to look for employment (probably the most legit reason out of the 3)
The second degree itself is next to useless unless it matches the job very specifically, its the other factors such as your marks in actuarial + personal qualities that help you compete against others when it comes to looking for a job.