Difference between 惊讶 and 吃惊？ Is there a difference in the usage between 惊讶 and 吃惊? I know you can say “令人吃惊“ and “阵脚我吃了惊“. 

Is there any way to divide the two?
yes, there is a way to tell the two apart. 


惊讶 is mostly used as an adjective, like 惊讶的表情 which means 'a surprising face look'. 
吃惊 is used as the same as 'be surprised', for example 我吃了一惊 which means 'I was surprised'.