From what I observed while playing this class for years, it seems that if you cast 2 venom or 2 pyro, it's much more likely that you don't get the stack than if you cast 1 pyro 1 venom, it looks like the chance stacks up from both but gets lowered only from one after gaining a stack.
It is very rare, Concentration r2 is enough to completely cover for it. With Elf racial in addition - that's just overkill.
True, i observed this too. atm i use Assasin with it, much to much con stacks, so i can use it, even if its not a really good spell.
pyro - venom - destab - thougthfull spirit - pyro - venom - destab and so on, and i nearly never have to use concentration.
CU
Grimpf