Organisation admins have full control over how they wish to collaborate and share their projects with any other organisation. All you need is the right email address of the eToolLCD user you wish to collaborate with and they can be added to any organisation. You can refer to the ‘Manage Users‘ section of this support post.
The more important question though is to figure out which organisation will ultimately hold the IP/ownership for the project so that you can decide which user will be added to which org. I.e. if Org A wishes to ‘own’ the Project in eToolLCD, you will need to invite the other ESD consultant to join Org A and then add them as a collaborator in the relevant Project’s structure. If Org A wishes to give the ownership of that project (assuming it’s already been modeled under Org A’s profile) to the other organisation, Org B will need to invite the creator of the project to their organisation and assign them a seat in Org B so that the project can be assigned/transferred to Org B’s profile & their users added as collaborators.
You will also need to juggle the seats around a bit as well to allow the users to do the required assignments (as collaborators or Project organisation owners) so you will need to contact the other organisation to communicate and collaborate on that. The support post above should have enough information on that to help you figure it out.
Let me know if you have any other questions.