There are two reasons why MFGRs don't recommend doing that.

1 - it takes twice as long

2 - There are a LOT of idiots out there (No offense if you're an idiot who happens to be reading this, but then again, you probably don't know it anyway) and it's just quite possible that one of these "entrprising individuals" will put the Joiner in place in one side and leave a huge blob of epoxy on the face of the Root Rib that will prevent the second wing half from seating properly.

So as long as YOU'RE not the guy the manual was written for, and you're not in a hurry, go right ahead.