Each Nightwatch account allows you to integrate with one master Google Analytics account.
If you have multiple Google Analytics accounts where you track different websites separately, you will need to decide which of them will be a master account and secondary accounts. Then, from within each of your secondary accounts, you'll need to give the master account access to their properties in Google Analytics.
Here's how you can give the master account access to the secondary accounts' properties:
From within a secondary account, click "Admin" at the bottom of the left sidebar,
Under "Property", click "Property User Management":
At the "Property permissions" page, click the "+" button at the top right corner:
Add the email address of the master account into it, with a minimum of "Read & Analyze" access:
Once the master account has access to all the secondary account's properties in Google Analytics, you can connect it to your Nightwatch account, and it'll be able to integrate website data from both the master and secondary Google Analytics accounts.
If all the properties from different websites populating your master account in Google Analytics can be confusing, you can also create two different Views: Client view (with all your client websites' properties) and Personal view (with all your personal websites' properties). Here are the steps to do it.