As much as we might want to still use a 'cracked stone' in the dry stone wall we are building but use it like 'two stones', because it is still the best fit we have to work with - and as much as it still is keeping to the rule of 'one over two and two over one' - as much as the combination is still structural, and the cracked stone is doing exactly the same job as two different stones would do - and as much as it would be really convenient to not have to look for a different solution and we know there are no stones that are going to fit as well - as much these things shouldn't matter at all and we say to ourselves 'who is ever going to notice anyway?' - it does matter and most people will notice, maybe not consciously but in some strange way a wall with even one purposely laid 'twin stone', unless it is a split stone with 'mirrored halves' laid together, not 'two ends' of a the same side of a stone, it is always going to look wrong. Don't ask me why, but the crack will not be seen as a normal joint. Maybe its because the crack will always be perceived as having happened after the wall was built and therefore it is sending a message to those studying the wall that something has moved and there are some uneven stresses on it. Maybe it is because the stone is giving away the fact that it has been laid along the wall not 'into it'. Anyway it is about as annoying to look at as it is to listen to someone cracking their knuckles. You can't really tell them why its wrong, but it is.