Separately or together?
For the lessons where you're introducing a new concept, skill or item (e.g. a harness) to your kitties, we recommend training them separately so that they can tune in fully to you and your cues without any distractions.
To separate your cats, complete each session in a closed room and keep them super short and sweet (around 5 mins) so that they don't have to be away from each other for too long!
For the cat who isn't currently being trained, you can give them an activity feeder or puzzle toy. That way, they're having fun too and they also know that the closed doors equal the arrival of fun foraging games, not boredom! You can then rotate which cat you bring with you into the closed room for training, with the 'activity feeder' station acting like your waiting room!
Once all your kitties have been introduced to a new concept, you may find that for more practice-style sessions you can train them all together (e.g. performing recall over longer distances, or being encouraged to explore movement in the harness once it is on).
Adapting your secondary reinforcer
If you are completing a course that requires a secondary reinforcer (a word or clicker to mark the desired action), you may wonder whether you should use the same reinforcer for every cat, or change them to ensure they are unique to each of your kitties.
The easiest option is to choose a different secondary reinforcer for each cat. For example, you can use a clicker with one, and a bell with the other. Or you can use a verbal cue and make it unique for each cat e.g. 'great job, Loki!' Just remember that your secondary reinforcer needs to be a sound or word that they don't hear outside of the training environment.