To quote the multiplier standards page:
"However, a good statement alone is not enough to get a high multiplier accepted. The amount of extra evidence one has to provide to get larger multipliers accepted is proportional to the size of the multiplier. For lower multipliers, like things much less than times 100, evidence can take the form of a clear increase in combat strength against priorly equal or superior opponents. For higher multipliers, like times 100 and above, the importance of stronger evidence, such as feats displaying power of a similar magnitude as the value the multiplier points to or the multipliers importance to the plot of the story, and a higher amount of evidence becomes increasingly necessary."
"If multiple multipliers are to be stacked, that are used upon each other, the evidence for the end result is equal to the total multiplier applied to the best feat. That means that if, for example, a character has a times 10 multiplier and later on gets another times 50 multiplier, than the evidence necessary to use both multipliers to get a statistic, is like that of a times 500 multiplier, as the best feat would be increased by a factor of 500 in that case."