I think on Google Domains you need to click a little + sign next to the existing TXT record so you can add another value to the same record, rather than adding a whole new record. The various DNS control panels vary a lot on how to do that.
We are requesting a cert for littleonejournal.com and for *.littleonejournal.com it's a quirk of ACME that validating both needs you to update the same _acme-challenge record at the same time.