Can you distribute an iOS app outside App Store using a single bundle identifier?
You should not be using an Enterprise Program membership in this way. The Enterprise Program should only be used to distribute apps to devices owned by the enterprise member company I.e. your company's devices. 

You can use a regular organisational Apple a Developer membership to distribute a B2B to your client organisations. 

You use App Store Connect as for a normal App Store app, but in the pricing and availability section you make it a private app and list the Apple Business Manager organisation IDs that are permitted to access the app. 

As well as being "the right way", B2B apps are lifetime signed like any other App Store app; you don't need to worry about refreshing distribution profiles and signing certificates periodically. 

There is a great WWDC 2019 session that covers this.