We are not able to enable both these voice assistants but are forced to choose. I'm sure I'm not the only one assuming Amazon, Google or both companies are actively preventing us from enabling both at the same time. I have however seen media reports that Sonos have publicly stated this is to enhance user experience. We'll probably never know the full reasons why this isn't currently possible.
I've created this thread so that those of us wishing for both to be enabled can clearly state that our user experience would be enhanced by being given the option to enable both assistants on the same device at the same time.