Hi Daisy,
Disclaimer first...I have never had pet rats in my life, but I have owned Degus for many years so this is how I would introduce a newbie.
In the first instance they are kept in separate but very close together so they can see and smell each other but not touch. [for around two weeks]
Degu's love to sand bath [like Chinchillas] so all the time I would swap baths and bedding....as you mention in your post they need to smell the same.
For introductions I put a towel in the bath [bit no water lol] and put on a pair of gardening gloves, then get a water spray on standby.
Then chuck them in!!!!
You're now going to see one of two things.... death or interest.
Use gloved hands to separate them if they really go for each other but remember you will see some showing off, squealing etc.
This is not an easy thing to do and can take weeks.... other Gu's I have re-homed I have been able to put in the cage with in days....It's a bit of trial and error.... Just keep the water spray and gloves handy at all times!
Lisa x