The answer is in the error message "there can only be one auto column"
Although I think, I know what you are trying to achieve, there is no workaround for DataAssist at the moment, at least I'm not aware of any. I hope DA3 will cover such a feature.
Interakt had this in their "MX Kollection" "Nextensio" lists and forms. The solution was to run a "transaction" right before creating a new database entry, which added an "auto-increment" value to a second column.
Take a look here for a detailed explanation: