Hi. I'm new here but a few days ago, I also had the same question about using Calibre on 2 different machines.
At the time, I looked through the forums and found the following thread, which was extremely helpful:
How to Duplicate My Calibre Library on Multiple Computers?
I posted this link because I thought it might be useful to you or others who might have overlooked it.
Thanks to that thread, I've been running Calibre successfully from my Macbook and IMac using Dropbox and am aware of what might cause potential issues (i.e. case sensitivity in Dropbox, running more than 1 calibre instance simultaneously, etc.)