The +double effect on accessories is probably additive to the base 8% (10% for awakened SSRs). Other listed +double/+triple effects on the wiki seem to work additively (the numbers they give make no sense if they were multiplicative). That said, it shouldn't be too hard to test, right? If you can stick 3 +double effects on one kamihime, you're comparing to see if the double attack rate is 14% or like 8.48%.