Training left-handed fighters in a predominantly right-handed world is a topic of endless conversation, particularly when it comes to weaponry. My teacher, grandmaster Angel Cabales, said to just teach lefties with the stick in the right hand to keep things simple. I've had many left-handed students over the years, and what I tell them is the weaker right hand will gain strength and coordination while their strong left hand will become proficient in checking, joint locking, disarming, etc., and for the most part that works. As Angel put it, each hand develops specialized skills, so neither is ignored. On the other hand, his son Vincent (and some other instructors in Serrada) teach left-handed students to use the weapon in their stronger hand, which may be simpler for them at the beginning but creates complications down the line. This debate is as old as military formations, where one person in line who is not in conformity with the rest not only stands out like a sore thumb, but in the old days of shield lines could create a dangerous gap for the enemy to break through. Even today in rifle drills and marching formations, everyone is expected to be on the same page.
I've seen similar debates when it comes to things like teaching guitar. There is no particular reason a left-handed guitarist needs to flip the instrument upside down, usually requiring modification of the instrument's bridge and restringing. One hand simply learns the fingerboard, the other how to strum or pluck; simple. It's exceptionally rare to find someone like Jimi Hendrix who could truly play ambidextrously regardless of how the guitar was strung.
Now in many FMA styles such as Serrada, there are drills such as lock-and-block or numerado where one training partner feeds strikes with either hand while the other practices counters, as well as double stick sinawallis which ostensibly train us to deal with an attack to either side of our body. That's all fine and good, but when it comes to sparring with weapons, it gets more complicated. Many are familiar with what the Inosanto/IMB folks call the "box drill" or sumbrada, a counter-for-counter exercise which, as far as I know, was introduced in western FMA practice by Angel Cabales. It is a symmetrical flow drill where each partner essentially is doing the same pattern, albeit 180 degrees out of sync. This teaches the shortest, most efficient counter to certain basic strikes, though at more advanced levels it becomes free flowing and any counter can be used. However, if one partner is left-handed and the other right-handed, this symmetry no longer exists and the pattern is irrevocably broken. As far as I can tell, there is no easy way to reconcile the two opposing sides, though it is possible to create and practice it as an asymmetrical exercise.
Recently two new students began training with me. One of them was born without a right hand, while the other is decidedly right-handed. This introduces the complexity of opposite handedness right at their fundamental level of training. While I can certainly go lefty myself to teach that individual, it is certainly more difficult for them to learn to practice together, especially as the right-handed one will have to learn twice as much as usual in the earliest stages. We're a long way from introducing them to counter-for-counter sparring, but I've already put some time into figuring out how that will work. Frankly, it's harder than the basic symmetrical drill, and will certainly create extra challenges if and when they reach that point in training.