Trainer/Admin Domain relationship

We are a high volume RTO that has split our volunteer and commercial training, and education departments into different domains to make it easier for our different teams. Unfortunately this has recently lead to an issue where our Volunteer Training Manager is unable to view the full list of trainers to work through their competency matrixes, evidence, etc.

Our trainers don't work across single domains, so do not have a domain listed on their user record, and we have been advised that this is the cause of the issue

The only resolution offered is that we need to remove his domain, which defeats the purpose of having our different areas set up in their own domains. 

Surely there is a better way to do this? It would be more logical that if the contact/user/trainer you are trying to look for does not have a domain then they can be seen by all domain admin users.

Alternatively, similar to another idea in this forum, Trainers should be able to be set up with multiple domains. Ideally this would be extended to Admin users who could be assigned to multiple domains as required without having duplicate records (which creates other issues)