Yes! You can install Highland 2 on as many computers as you have linked to your Apple ID. The Pro upgrade is attached to your Apple ID, so it will carry over to any additional computers you install it on.
To install it on a second computer, go to the Mac App Store and sign in using your Apple ID. Click the Account tab, and you’ll see a list of all the apps you’ve purchased, including Highland.
From there, just click on the Install button. It will download, free of charge.