Getting scared half to death twice can be interpreted mathematically. On getting scared half to death first time means getting scared to death half once or 50% or 1/2 times alive.
On getting scared half to death second time implies that being scared half of the remaining life which is 1/2 of 50% or 25% (1/4 in terms of fraction).
Hence, you are scared to death by 50% +25% =75% or 3/4 in terms of fraction.
Getting scared twice half to death implies you are 25% or 1/4 times alive.