How about quarantine? Do you really want to jeopardize the health of your current animal by putting another one in it's tank?

How about upgrading your 4 feet female to a bigger enclosure (20 gallons is a tight small for her), and give the small tank to the new addition and quarantining them as they should?

Also you realize that if you were to house them together (which is not recommended) you need to be prepared for what's to come.