The place of supply of a service is determined by where the customer belongs for a particular supply. If they have separate branches it can be difficult to decide where the customers belongs for a particular scenario. However both parties would need to be satified as to where this is.
If, as either the supplier or the recipient of services, you have establishments in more than one country, it is essential to determine which of those places is treated as making or receiving the supply in question.
It is generally the case that the business establishment will be the most closely concerned with the supply. But, if the supply is made using the human and technical resource of a fixed establishment, or the supply is received for the needs of the fixed establishment rather than the business as a whole, then this will be the place most closely concerned with the supply.
Where a branch of a company contracts and pays for purchases that are to be used by the whole business, the supply will be seen as being made to the business establishment, not the branch. This will be the case even where the branch creates an internal charge to the business and other fixed establishments.
Please see the guidance which helps determing where a customer belongs where they have more than one establishment:
Place of supply of services (VAT Notice 741A)