PADI sidemount is much of a joke as the instrutor teaching it. Where I have respect for PADI,there are instructors who will learn sidemount with a course director one weekend,and declare themselves a sidemount specialty instructor the next weekend. If you have an instructor with a lot of experience,then this could be worthy. It is worthwhile to be comfortable in the configuration that you use most when taking cavern,otherwise like when someone shows up in doubles the first day of class for the first time,it is a true charlie foxtrot. If you want to be in sidemount configuration for a cavern class,then perhaps take that sidemount course,get very comfortable in that configuration,and then plan on having your set up tweaked by your cavern instructor. I think trying to learn sidemount and cavern at the same time may not be real optimal because you may be concentrating on new gear configuration issues,that you miss out on the nuances of the class (visa versa).