macOS High Sierra: Can't use Mouse and MacBook Trackpad at the same time
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!!!