Hello @Maura19,
There isn't a way to do that, at themoment.
Currently, co-hosts can be selected only per listing as the "co-host" setting is on the listing's calendar, not on the host's profile.
You could try posting a suggestion for this feature on host voice.
When you're on this forum click on "Discuss" at the top and then on "Host voice". That's an area where people post suggestions for Airbnb. Others can then vote on the suggestion and Airbnb says that they will consider suggestions that get enough votes.
Steve.