macOS High Sierra: Can't use Mouse and MacBook Trackpad at the same time Using a Mouse at the same time as being able to use the built-in Trackpad of a MacBook has worked in previous versions of macOS. But with macOS High Sierra, 10.13.0, they don't work in parallel anymore.

The issue exists both with all types of mice, including Bluetooth and USB, and third-party as well as Apple's original mice.

Is this a bug/issue or perhaps a new "feature", and can it be remedied?
I had the same problem, but find this solution that is working for me:

Go to System Preferences >> Accessibility >> Mouse and trackpad

Here, un-check the option "Ignore the integrated trackpad when a wireless trackpad or mouse is connected"... That's all!!!

I have my OS in Spanish, therefore I am not sure about the translation for the English version, but I really hope that this can help to solve your problem. 

Best regards!!!