Goro borrows his sister's power and it is not additive he switches completely to Sacred style when he does that and is at 5000+ ap his purple skin is attributed to having storm as his element even though it is actually due to Kuro.
Kagura doesn't get Goro's power as she is completely vulnerable tho goro borrows her and also has his own basically at the back to switch back Goro sets traps with his cursed style before fighting and then switches to Aoyu to fight most of the traps would be activated via some talismans or item with Goro 's energy in it so he doesn't need to switch back to Kuro to use it that said It is likely the strongest traps will need him to switch back to kuro to activate them before going back Aoyu.
So likely Goro's coin helped in whatever trap he had set for you as he punched you with his aoyu strength but his coin or whatever thing he uses to activate the trap was also used which is why you were just sent flying that was his Cursed trap working.
Ap potential wise Kagura and Goro are pretty much same as in innate talent but this ability of goro is what separates him and this is what made him the heir.
The power simply cannot be additive because it is two different styles and what not but yeah there you go that is how it goes with goro and he has high Mastery over both styles.
We don't know if it is a bloodline thing and or unique to Goro only and even if it was a bloodline thing he only manifested it but whatever it is it made him the heir.He was highly talented without it anyways too.