Kralkatorrik showed he could use other dragon's magic, but it also caused him a lot of pain. So it makes me wonder how does the Elder Dragons' magic works against each other?
Primordus and Jormag's magic are against each other for sure.
I guess Zhaitan's magic didn't work too well on Mordremoth, too. Even worse than Kralkatorrik, which is why Zhaitan's death didn't awake Mordremoth and he didn't get too much benefit from Zhaitan's power.