Why aren’t light side points farmable in campaign but dark side is?

Yup, you have to select one or the other.  You can't have 2 (or more) at the same time.  The good news is that there is no limit on how often you change your Journey.  So, farm your DS, then swap your Journey and farm your LS.

A little advice:  At the end of your farming make sure you swap it back.  This way, you get into the routine of it and don't waste energy.

You can’t farm the currency unless it’s associated with the GL event selected. I assume you have a DS GL selected, yes?