There are only a few cases that you have a slayer on a weapon AND you have a slayer on a Talisman.
They definitely do stack. Like the undead case above, which I know is true. Also, ratmen slayers and repond stack together.
The reason you don't see it much is because of all the other stuff we build up on that 300% side. We're almost always at the cap all the time, so changing something won't really make much of a difference.
The way to test it is to NOT use EoO, Honor (perfection) or anything else. Just go basic with a slayer weapon. Hit a few times then put on the slayer talisman. If your under the 300% you will see an increase in damage.