I suppose that might be an option, but switching back and forth between the two system photo libraries could be confusing and tedious. (You can only designate one library as the system library at a time.) Each switch would probably require a complete re-sync/merge.
Some users have chosen to have a small system photo library on the internal drive synced with iCloud and a larger complete library on an external drive. You would then have to pick and choose which photos and videos should be in the smaller library. (Maybe just favorites, maybe just recent photos, maybe just business or just travel.) Depending on how photos enter your libraries, you would need to copy new photos in one direction or the other. (Photos arriving via iCloud from an iPhone would go to the synced library; you would copy them all to the larger library and remove the ones you don't want online all the time. Photos imported directly from cameras to the MacBook would have to be copied to the smaller on as desired.)
There is a more exotic path that should work if you are comfortable juggling multiple Mac users.
Had you considered just viewing your iCloud Photo Library on the iCloud.com website with a browser when you cannot attach the external drive?