LrMobile syncs with only one catalogue. So to sync LrMobile to your MacBook, you would need the synced catalogue to be on it.
But what you can do on your MacBook is use your web browser to access the photos which you synced to LrMobile. Log in at lightroom.adobe.com ("Lightroom Web") and you can do some adjustments, crop, add stars and flags. You can also drag files from Finder on your MacBook and drop them in the Lightroom Web browser window. They will be uploaded to LrMobile and then appear in your synced catalogue.
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Simple fact is you can not Sync two different computer with LR Mobile whether or not you use the same catalog on both computers.
LR Mobile was created to sync one catalog from one computer to many iOS or Android mobile devices. If you copy your already synced catalog to your other Mac and Sync it up with LR Mobile you will be Un-Syncing the original computer and that same catalog on the original computer from LR Mobile.
This is a point I have tried to stress in many threads. LR Mobile only works with One computer and One LR catalog on that One computer. Sure you can copy that same catalog to 2-5-50 computer and Sync them up with LR Mobile. But each time you sync up a different computer you Un-Sync the computer it was synced with before.
One computer and One catalog at any given time.