It is very easy to smack a horse wiht joined reins as you can get all different rein lenghts. You can shoulder smack easily with them no bother, I believe wild_spot mentioned she did the same on her eventer. You just flip them over the neck simply.
I actually ride in long, plaited rope split reins, for many reasons, one of the last being that they can be used in lieu of a crop.
My eventer was never hit in any way when being ridden, as I described, I think he would have reacted very badly.
However it is quite easy to over under with buckled reins - if you have the coordination to carry a crop and somehow use it behind the leg without altering your position, then you have enough coordination to use your reins on the shoulder.