Thank you for posting your query on Microsoft Entra ID. From above description I could understand that you are looking to setup 2 workforce tenants and have B2B collaboration with customer 1 and 2 respectively. Also, you want to allow user from your tenant to access the apps in both the tenants.
Please do correct me if this not the ask by responding in the comments section.
You need paid Microsoft license plan to create an additional Microsoft Entra tenant (workforce). Microsoft paid cloud services, such as Office 365, EMS require licenses. These licenses are assigned to each user who needs access to the services. Microsoft Entra ID is the underlying infrastructure that supports identity management for all Microsoft cloud services and stores information about license assignment states for users.
The billing for each external user would be done MAU basis.
In your Microsoft Entra tenant, guest user collaboration usage is billed based on the count of unique guest users with authentication activity within a calendar month. This model replaces the 1:5 ratio billing model, which allowed up to five guest users for each Microsoft Entra ID P1 or P2 license in your tenant. When your tenant is linked to a subscription and you use External ID features to collaborate with guest users, you're automatically billed using the MAU-based billing model.
- So only one workforce tenant with P1 or P2 plan would be needed.
If you don't have any further queries and the suggestion above answers your ask, please "Accept the answer", This will help us and others in the community as well.
Thanks,
Akshay Kaushik