When to introduce male to female?

I had two female rabbits who were fantastic together, both were spayed young. At 2 years old one of them died. Devastated, she was the crazy one who ran about the house, hiding under our beds. But we didnt want to leave her sister Marshmallow alone so we rehomed a male, who we have named Drax. Drax is 18 months old but is not neutered. He is getting neutered next week.

Right now Drax and Marshmallow are in adjoining enclosures, happily sniffing at each otger and sharing food through the bars. After the neutering when should we introduce them to each other without the divide?

Thank you in advance!