Subbing! My horse will also sometimes cross-fire. What I do is immediately transition back down to a walk and try again. We are also using the method where one person is in the middle of the roundpen giving the canter cues, while the other (me) is on Brandy's back giving the riding cues. It is working well but I like to keep my eye on threads like this for extra advice and ideas.
**I must not forget to thank the difficult horses, who made my life miserable, but who were better teachers than the well-behaved school horses who raised no problems.**