Silly B2, if men can do it, how hard can it be?!?

Get two of these for each bookcase, including the bottom ones.



Drill a straight pilot hole near the top rungs of each vertical support. Put a screw through the pilot holes and put the case where you want it to go. Tap the front so you make a mark in the drywall.

Now move the case and make a starter pilot in the drywall, then manually screw in the drywall anchor at a very slightly downward angle. Install the bottom ones by manually screwing those pilots into the anchors.

When they're stacked and installed, drill two pilots through the top case's base and 1/3 of the way into the head frame of the bottom case. Shoot screws through to fasten them lightly and then load 'em up.

Easy peasy.

If you hit a stud, skip the anchor and go for the screw.

Sorry, B2, I assumed facts not in evidence. Your original Q: Stack them only after they're fastened. I presumed you required help because you were installing them in a location where studs would be unavailable to your scheme.