I previously had 2 male budgies. They were the best of friends and shared a close bond. When Pix passed, Twigs was a lone budgie for a while.
This is the way I did it when I decided to get Mink (female)... I got them a double flight cage with the option of a cage divider. After quarantine, both Twigs and Mink moved into their 'mansion'. They got along well together from the beginning, but are definitely not a pair. Twigs was way more smitten with his late male friend than he is with Mink. Still, I keep the environment in such a way as to discourage nesting and breeding mode just in case they decide to one day become more 'friendly'.