@Oli R Thank you for reaching out to us, As I understand you have signed up for Azure subscription and created a new tenant (tenant A) while activating a P2 license you created another tenant (tenant B) which has P2 license.
I would recommend this approach create a new user in tenant B (******@tenantB.onmicrosoft.com) add the same in tenant A via invitation, make the sample user as Global admin & Owner rights at the subscription level and need to do switch directory once you have the user added as guest.
Reference:
https://learn.microsoft.com/en-us/entra/external-id/b2b-quickstart-add-guest-users-portal
Let me know if this approach helps to resolve your issue, else send me an email to 'AzCommunity@microsoft.com' with Sub - Attn: Givary and following details in the email body:
Link to this thread/post
We can connect offline and discuss further on this.