Join Date: Sep 2011
Location: Germany- but not German =D
I have never used a whistle, or seen one being used. The trainer, during practice, would act as a caller. If still required, would call when we ride for the 'show'.
You just have to be aware of your partner horse, and the horses behind you. Leave enough room, and be able to speed up or slow down when required. If you're both doing a turn together, then naturally the horse on the inside will have to slow down where the horse on the outside may have to shift a bit more to keep up.
There are many things you can do, but I would keep it simple until you are all managing to keep an eye on each other and staying at the same pace and distances, then add more difficult things in.