I would suggest you to install Lightroom on both of the Macbooks and keep your Lightroom Catalog and photos on external drive.
Use the same catalog on both the Macs to keep your work synced up.
Thank you Sarika.
Can I install Lightroom on both macbooks with only 1 license?
Yes, It can be installed on two computer but you can work on one at a time.
Thanks Sarika. I should have mentioned that I have Lightroom 5. Can I assume your answer would be no different?