Of course, most famously, they also have the pouch. The pouch is like an external auxiliary uterus, in addition to the two interior uteruses. Kangaroos do not mess around when it comes to female reproductive organs. So here's how it works. When the doe is feeling ready for sex, she'll give off a scent to the bucks. Kangaroos live in groups, called mobs, that have complex social structures that differ by species. So okay, when a lady is feeling it, one of the guys follows her around, stroking her tail and smelling her pee, until it's go time. Sex for a kangaroo lasts 10 to 15 minutes.
Sperm goes up a side tube to meet an egg hanging out in one uterus. That fertilized embryo will grow for about 33 days, pop out, blind and hairless, and find its way into the pouch. It nurses there, on one of mom's four nipples, while mom goes out and gets knocked up again. This embryo either develops or not, depending on whether the mom kangaroo wants it to. She can just keep it there until she needs it. Maybe Todd Akin is actually a kangaroo? That would explain a lot.
Once the first joey is starting to go out of the pouch and explore around, its sibling pops out and crawls into the pouch to start developing, and mom can get knocked up again. Kangaroos can have up to three joeys going at once. It's basically like having one of those Park Slope triple decker strollers, but on the inside. She can make different milk for each joey depending on how many calories it currently needs. She can even determine the sex of the babies! Kangaroos are level eleven fertility mages, in control of every aspect of baby-having at all times. An heir and a spare doesn't even begin to describe it.