I usually post process images on the iMac in my study or in the field on a MacBook. Occasionally I need to begin the process on one machine and finish it on the other.
Exporting and importing catalogs between machines is very easy in Lightroom and works well when you have done all the post processing on a single machine and just need to take everything across to the other machine. It is a little more complicated when the images already exist in catalogs on both computers, and you have made post processing changes in both places. This is where I’ve found the XMP sidecar files come in useful.
To apply changes I have made on the MacBook to the same images in the catalog on the iMac I select all the changed images in the Library view of Lightroom on the MacBook and choose the Save metadata to File command from the Metadata menu. This creates the .xmp sidecar files in the same directory as the master image files. I can then copy these files to the equivalent image directory on the iMac. On that machine I issue the Read metadata from Files command also from the Metadata menu and all the tweaks made on the MacBook are imported and applied to the images on the iMac. The settings being saved in the catalog database on the iMac.
As Lightroom is cross platform this will also work of course, when syncing changes between a PC and a Mac.