Develop in-house app for another company signing with own Apple Developer Enterprise account
Apple does not allow you to use company A's Enterprise account for signing apps to be distributed and used amongst employees of company B, unless company B is owned by company A. 

A few other exceptions exist, for example for car manufacturers and their dealerships, and for hotel chains and their individual hotels. Also an exception exists if the employees of company B are actually physically located at company A's premises, and company A control the devices (i.e. they own the devices or they can remove these apps again). If your case do not fit these exceptions, you'll need individual approval from Apple for you case. This is normally not possible to get.