I've used a couple of weapons now that deal more than one kind of damage (not counting neutral), like the silversix or the sun shards. These weapons never get the super effective damage indicator, even though logically they should be strongly damaging to fiends and undead, neutral to beasts and constructs and resisted by slimes and gremlins.
The numbers are grey for slimes, gremlins, beasts, constructs, and appears neutral towards fiends and undead.
This is kind of a small thing but I like to be able to see that i'm not doing something stupid.
Accurate Damage Display
Check the options tab. You'd be surprised what things can be changed in there.
Theres still a bug concerning damage effects with several monsters, but as a rule, any weapon that does split damage shows grey if a monster is resistant to either damage type. The super effective hit indicator is also tied to the amount of damage dealt to a monster. I'm not positive, but it looks to be at least partially qualified by doing +X% damage over a base stat of the weapon.
You'll notice it with single charge attacks on some weapons against monsters that aren't weak to the damage type, and frequently on weapons with a chained charge attack (like the Antiguas). I've even seen it happen with the charge swing of the DA against fiends and even Wolvers on rare occasions.
Those weapons like silversix will never be "super effective" because half the damage is good againts a certain moster but the other half is weak, thus cancelling the effect.
SO if you have a double damage type, and you fight someone with a defense against one, but is weak to the other, you only get the WEAKER damage?!?!?!?!
If you just want to see the actual numbers of what damage you are doing that is in the options menu (press escape)