Just make sure the RW project file is not open simultaneously on both the MacBook and the iMac. Otherwise you might get a ‘sync conflict’ and the project file can become corrupt - with one version ahead of the other.
RapidWeaver project files are inherently quite fragile. Breakage can occur if the file does not fully sync - like the computer goes to sleep or is shutdown before a sync completes - risking some parts of the project file missing or being further ahead than other parts.
Ensure that you have a fast and stable internet connection that allows the changes to sync-back to Dropbox quickly and reliably.
I no longer have Dropbox installed on my computer, because I was growing concerned about all the ‘processes’ that were running in the background and the root-level access Dropbox needs to read files and install automatic updates. I now only use the web interface of Dropbox for sharing files with people by email.
To sync project files between computers, I simply use a flash stick. More recently I discovered my PlusNet router has a USB port on it, and plugging-in a flash stick (formatted in Windows FAT format) makes it work as a simple NAS (network attached storage) disk, that any device in the house and see and access files from.