This is an intriguing idea that you should submit to the idea board once it reopens. It sounds like something you can do in Excel, where a cell can "look" to another cell and update when that other cell is updated.
At this point, though, I wonder if it would be easier to use the LAB course as informational only and just tell the students to look to the LECTURE course for all their grades, including lab grades. Then just use the Lab Assignment Group in the LECTURE course for all the individual grades and not just the final lab grade as an import. (This is assuming you don't have to report the LAB grade separately as well, per school policy. I'm assuming these courses are created for you by your institution, else you would do it all in one course from the get-go.)
I think it is possible to import grades to Canvas via a csv file, but that seems like more trouble than it's worth for what you are wanting to do.