Kemo Sabe, (is mixed case OK?)
You are not the first to want such a feature. Lightroom's handling of raw+jpeg leaves some things to be desired. Because of how Lightroom works (importing both does NOT require significantly more storage), most people think it's not such a big deal - if not enough room on laptop for raws, then put them on external drive instead. If you want raw development capability when offline, develop the smart previews until raws are online.
That said, I have developed a (free) plugin called "Raw Plus Jpeg". It has the feature to import jpegs corresponding to already imported raws, but at the time of this writing, does not have the feature to import raws corresponding to already imported jpegs. I have received multiple requests to augment it with the feature to import the raws for the jpegs, and will be implementing that feature soon.
I don't know if you'd be willing to use a plugin for such capability, but it'll be available soon as an option if you want.
Rob