Does it make sense to vary the inspiratory and expiratory part of Respiratoy Muuscle Training (ie do inspiratory or expiratory only)?
In most cases it is beneficial to train both parts of the breath cycle.
For example, if your inspiratory muscles get stronger by doing inspiratory muscle training and you start increasing your exercise tolerance and being more active, you also want your expiratory muscles to be strong, to then excrete the CO2 that you're producing, because expiration usually is just passive. So it's really good to get those muscles strong as well.
Or for example, if you want to improve your cough effectiveness, you need really strong expiratory muscles, but you also need really strong inspiratory muscles to draw the air in before you produce an effective cough.