Have patience! If they only met one day ago, they’re still a little unsure of each other. Your female is in her own home – so at the moment she sees the male as an intruder. She doesn’t want to lose her home, so she’s warning him off.
Don’t leave them alone together for now, just keep an eye on things, but I’ll bet in a couple of days the hissing will have stopped, and they’ll begin to play together, eat together and – before too long – you’ll find them curled up together for a sleep.
In my experience the girls are a bit less keen to share their home with a new boy, while the boys seem quite content to add to to the family. But this is only a temporary thing. They are both young enough to become good friends, but it won’t happen the moment they set eyes on one another. A bit of patience, and keep on treating them both the same as ever. Feed them, cuddle them, talk to them. And stroke them both, swapping your hands over so that they each receive some of the other’s scent.
Things will almost certainly settle down soon. Best of luck with your family!