ok, this may answer the question or not:
I took an old black double slayer bow ("of orc slaying") with
1. snow elemental slayer
2. orc slayer
(they have no other mods)
PoFed it to max (just in case I can make something nice out of this) and tried.
First when trying any kind of super slayer (in the menu) it always said it would replace the first one (being snow ele).
I imbued reptile slayer and was then left with a reptile/orc double slayer.
When I went to the menu again and tried to imbue super slayers and slayers it showed that the first property (being reptile) would be overwritten.
So actually the experiment is over but I thought I try something funny:
went out and killed orcs with the bow as it is. My leg archer did about 100 - 120 damage to normal orcs (one hit kill) and around 80 to orc brute (i guess orc slayer should work against brute).
Then I thought I imbue orc slayer to get an orc/orc slayer and see if it actually counts double (not that orc/orc is useful of course). Was simply curious.
BUT, once orc slayer was imbued, only one slayer remained on the bow, so orc/orc was not possible.
After this I imbued repond to see if the second slayer property would somehow reappear but it didnt and so I am left with "a bow of orc slaying" that has only one property: repond.
2 PoF, 10 white pearls, 10 relics, 10 goblin blood wasted but hey, it was fun
Hope this answers some question though...
Hephaistos of Europa