How do I run my Vue client and Express API from the same script command?

Question

I'm looking to run my two apps at the same time from a single script in the main directory's package.json.

How would I accomplish this?

So far I've come across using && to do something like cd api && npm run start && cd client && npm run serve and that only results in one of the applications running without the other.

My folder structure looks like:

MainDirectory
|-Package.json
|-API
|---Package.json
|-Client
|---Package.json