Well, two doublings make a triple, so if it happened to stack with Keen on a 20/x2 it would become 19-20 on a Keen and then 18-20 with that ability.
Because it now increases the threat range by one you should state whether or not it stacks with other effects that increase crit range and if it does, how. Vague or missing wording often leads to... well, not broken things, but not pleasant areas.
Also, do you have anything planned for what an Epic Progression might look like?