how to manage Multiple User Acceptance Tests at once

Our current setup has the following environments:

• development
• testing
• staging
• production

We are finding that things are being held back at the testing phase due to UAT taking longer than expected due to time available of the user.

I was thinking that creating multiple test environments to deploy to might be the answer, but I wanted to see if there was a more standard approach to handling multiple user acceptance test at once withing a testing environment.

Cheers, David