Explode... Go off... I don't know what to call it
R2 would be a little awkward, because most likely, just 1 part of it would explode, leaving the 2nd part on still...
But then again, it also makes it so that you still have "magic" on target, despite some of it wore off from the explosion.
(Also R2 might have been to good with such a tome, if both parts went off at once, so basicly it fits well in to current mechanics in a way)
Don't know if the "random bursts" could be a problem. There are times when you want to hold your DPS, and this could be an issue, but if the dot is on, the DPS isen't held anyway...
I think it could add alot of fun to the class, ofc more dps is always fun. Though I'm pretty sure that it would take some time to get "procs" as you need atleast 2 dots on target, and once one goes off, well you lost an elemental type, so have to re-apply, and obv they also just wears off after time, so also have to chose what dot to do, ect.
"Hm, R2 Pet-heal just went off, should I do R2 again, or should I recast the poison that just wore off...."
"Hm, 2 targets, should I cold both, or should I cold and fire 1 target first, and then start building on the other..."
"Hm, I already have 3 elemental types going, should I cast poison for the stable high dps, or should I cast disease and hope it goes off early..."
It would be a little like Wizards primals, just not as good, but still similar in small chance of "bigger" bang. Necro's would just have to work harder for it to even being possible, and also hope that when it did happen, it wasn't the very last dot tick of the dot