Both of the conjunctions 甚至于 and 以至于 have the same meaning “to the extent that…”. Such sentences should have two parts.
In the first part we should describe the situation. In the beginning of the second part we should state the extreme result.
We usually use 甚至于 when we talk about a negative result. The person doing the action is to blame for the result.
Bank officials are usually so calm to the extent that they are a bit boring
zhè jiā yínháng de guānyuán tōngcháng shì lěngjìng de, shènzhìyú yǒudiǎn wúqù
He keep talking all the time. He can't be silent even for a second