I feel this is two questions - the etymology of 'double edged sword' as a metaphor and 'have swords historically had two edges as default', so I'll attempt to answer it that way, but I promise to link them up at the end. Let's look at swords first, because metaphor attempts to explain something using an artefact of a shared environment of cultural understanding - so we must set some parameters around that environment to understand the metaphor.

We're going to go broad, here - we'll get granular when it becomes relevant. Ancient European swords were typically double edged. The most produced would be the Roman Gladius and related designs (pre-modern design tended to be iterative, the Gladius evolved from other weapons and other weapons evolved from it. But we can take one example as a snapshot along the iteration process.) These swords featured a sharp point for stabbing, a very basic handguard, and two sharpened edges for cutting and parrying. The point is helpful for penetrating armour.

By the middle ages swords had become more complex, with a growing trend in Europe, North Africa, and West Asia for handguards to have a 'front' and 'back', even if the blade remained symmetrical and sharpened on both sides. This created a de-facto 'front' edge, which didn't have a monopoly on cutting but it would be the default. The point is very important now, for exploiting weaknesses in armour of all types.

Now we get in to a period called the 'Military Revolution'. Later medieval and early early modern swords start to see more complex guards that lock you in to one edge as your leading edge. You now favour this edge for cutting. Simultaneously we see a decline in armour (which is another discussion for another time) which brings back cutting!

As we get into the Early Modern period we start to see specialist stabbing and cutting swords. The rapier is a very precise stabbing weapon, able to reach out a long distance and drive deep into organs through the tiniest gap in armour or other defence (such as a buckler or rival's sword - remember swords are defensive as well as offensive. They can parry.) Rapiers can have two sharpened edges, one, or even none! But the most common was to sharpen both edges at least some of the way down the length.

Parallel to this, we see weapons such as sabres become very popular in certain settings (especially cavalry), which have a curved final section of blade and are biased towards cutting. Typically they will have a leading edge sharpened all the way down to the hilt and a back edge only sharpened a little bit (just in case you want to stab a bit.) They typically have a complex guard that sets its orientation and protects your hand - it has a clearly defined front and back.

Later into the Early Modern period and as we get into the Modern period we see a continued proliferation of sword types. Literally hundreds of distinct types each with much variation. Rapiers and other infantry-focused swords fall out of favour as the musket and bayonet take over. Bayonets can be simple knives stuck in the barrel (plug bayonets), spikes focused on stabbing (very common in the 18th and 19th centuries), or sword bayonets.

Sabres remain popular, and are joined by other cavalry-specialist swords. We start to see stab-focused cavalry swords designed to be used almost like a lance, many only have one sharpened edge. Some have none and are a simple spike!

It is in this environment that we start to see 'double edged sword' appear in English (the term had been in use in Arabic for some time already - it's worth noting that the Arab-speaking world knew how to exploit a single-edged blade for some centuries ahead of Western Europe. But there is no concrete etymological link that I know of.) But the term is almost always used as part of an explained metaphor. 'His logic, like a double edged sword, cuts both ways'.

We still describe something as 'cutting both ways' and still use 'like a double edged sword' - but they have been orphaned from one another by the constant shifting of the English language. Originally, we would have used the terms in conjunction with one another as one complete metaphor. Now, let's bring our metaphor and our environment together:

And, at that time, swords were sometimes double edged sometimes not. So for the cultural landscape that the people using the term lived in, it was a helpful metaphor. Today, swords are not a constant part of our culture (well, I work in a military museum so it's a constant part of mine. But that's only partially my fault.) so the metaphor only continues to work because we're already familiar with it. But because we're already familiar with it, we don't need the other half of the metaphor.