I have a recent Imac and an apple macbook pro. I have Lightroom 6 installed on both and use an external disk drive to keep my photos. My catalog files are stored on the computers.
How do I easily use on or the other computers to update my most recent catalog on both. Sometimes I use my bigger iMac and sometimes I use my Macbook pro.
I have type latest version of El capitan on both computers.
If you continue to maintain catalogs separately on each computer then it will be necessary for you to copy the catalog that you used last to the other computer. This will require you to keep track of which computer was used last. The easier solution would be to copy the catalog and the previews folder to the external hard drive and use the single catalog.
Please follow the steps provided in the below link to transfer Lightroom from one computer to another.
Also, I would suggest you to keep the catalog and the images on the external hard drive based on your work flow.
This will maintain a main catalog and the images which you can access to any of the computer.
Hope this helps.