双刃剑 vs。 两面刀 which is used more often? I'd like to know which of these two phrases you use more often. I know that in general they both mean "a double edged sword" is one used more in Taiwan or in mainland China?
刀, by definition, usually has a curved blade with one sharpened edge. Thus a Samurai's sword, a scimitar, and a machete all fall into the definition of 刀. 

If it has a double edge, and it has a straight shape, it is usually a 剑.

双刃剑 is the standard translation of double-edged sword. There is no usage difference between Taiwan and the mainland.

In very rare cases, 两面刀 is short for 两面三刀, which describes a two-faced person who is secretly trying to harm the other.