Not sure if there’s a misunderstanding here.
Because buffs stack onto the previous (or current depending on your point of view) value you cannot add percentages like that. E.g. 100% + 2% + 2% = 104% -> 4% increase.
Unless L2 does something really weird or smart the second buff would simply use the already buffed value as base value. So you would get a bonus on the bonus. Like 100 + 2% = 102% -> 102% + 2% = 104.04% -> 4.04% increase. Which means you should notice a 36.64% casting speed increase boost of the 34% you calculated.
I can’t really comment on possible changes at the moment. This quite simply hasn’t been tested well enough yet. If we should notice serious problems once we have more PvP fights we sure will address them. At this time it’s too early though. But it’s good you’re actively thinking about this kind of stuff and making us aware of potential issues.