And Coley says he can't wait to see them grow up together. He's hoping the brothers will be best mates.
There were 1774 twin births throughout New Zealand last year. That's more than 3500 little babies who will grow up sharing clothes, a room, mannerisms and, to one degree or other, looks. That's 3500 children who will, on the most part, have a special bond with their sibling that even their parents can never quite understand.
The number of twin births roughly doubled through the 1980s and 1990s, but has levelled off since in vitro fertilisation specialists changed their approach to multiple embryo transfers. Five years ago, 25 per cent of IVF babies were twins; today that number has dropped to 8 per cent.
Fertility Associates head embryologist John Peek says that is a deliberate move. New Zealand has a policy of trying to produce singleton babies rather than multiple births, because of the health consequences for the babies and their mother.
In New Zealand clinics, 85 per cent of women aged 37 and younger now have only one embryo transferred at a time. This is much higher than most countries; in Sweden, the rate is 70 per cent, in Norway 53 per cent and in Lithuania 3 per cent.
Peek says in other countries, such as the United States, multiple transfers are done for commercial reasons. The increased chance of pregnancy gives better statistics for advertising.
But in New Zealand, publicly funded IVF treatment rules prohibit more than one embryo being transferred at a time.
Multiple births come with increased risks of complications and miscarriage. Babies are generally born much earlier and often at dangerously low weights.
But for older women, more embryos are transplanted because the chance of pregnancy overall is much lower.
"Younger women have one back at a time because the chance of twins is so high," Peek says. "For women in their late 30s, two increases the chance of any pregnancy quite a lot."
In the past, people tended to ask for more than one embryo to be implanted at once so that they would have an "instant family" and not have to go through the IVF process again.
But Peek says that is less of a concern these days. The survival rates of embryos that are frozen and thawed is so good that few women have to go through the egg-harvesting process more than once, which is gruelling.
Whether they are the result of IVF or a natural occurrence, twins seem to capture the imagination.
Babies in matching outfits seem more than doubly cute, and twins who physically feel each other's pain, and then finish each other's sentences when they talk about it, are the stuff chat-show dreams are made of.
Ashley and Mary-Kate Olsen, the annoyingly cute sitcom actress toddlers who turned into surly, pouting fashionistas, have much more star power as a double act than they could have harnessed individually.
Kiwi actress Anna Paquin gave birth to her twins this week, a few weeks early. Already, women's magazines are clamouring for pictures of the pair.
Websites, magazines, clubs and societies have been set up to celebrate and support families with multiple births. In the mid-1990s there was a lot of publicity about the pain the death of a twin in the uterus could cause the survivor, and pop psychology blamed some cases of depression on "vanishing twin syndrome" where two eggs were fertilised together but one, not viable, was reabsorbed into the mother's body, sometimes before it was even noticed.
But Fiona Purchas, president of the New Zealand Multiple Birth Association, says there are a lot of misconceptions about twins, from the idea that all are the result of IVF to the expression "double trouble" that seems to fall out of people's mouths as soon as they encounter twins.
"A lot of parents find that tricky to deal with," Purchas says. She does concede that some toddler twins can be known to get up to mischief together, working as a team. "They are a lot of extra work but they're not double trouble. We like to say double the love."
Purchas says old wives' tales - like the belief that twins grow up to be more competitive adults - are myths. But scientists are discovering that twins do have, in some respects, measurable epigenetic differences from their older or younger siblings.
In world-first research, a team at Auckland's Liggins Institute have found that twins are more prone to obesity and diabetes.
Associate professor Frank Bloomfield says twins, overall, are born significantly earlier than singleton babies - on average at 36 weeks' gestation rather than 40 - and are also smaller at first, even taking into account their earlier birth.
"It has long been thought that this is due to the constraints of intrauterine space and competition for nutrient supply between the twin pairs, but there is little evidence for this."
In experiments involving pregnant sheep, Bloomfield and his colleagues found that where ewes conceived twins but had their pregnancies reduced, the remaining baby continued to behave as a twin.
"They were born at a similar time and the lambs were more similar in size to twins than to singletons, even though they were born as singletons. Furthermore, when they grew up to adulthood, both the twins and the 'reduced' twins who, of course, were born as singletons, had increased fat mass compared with singletons."
He says this could be because of local environmental factors in the reproductive tract. "In the case of twins, this may be ... embryos 'talking' to each other or signals between mother and embryo that then result in altered developmental trajectory leading to earlier birth and smaller size at birth."
Mum of twin girls, Charis Robinson, is painting with them on the deck of her Auckland home when we call.
In between requests to her girls, Lily and Isla, to not go inside covered in paint, to not drink something they shouldn't and to leave what sounds like the cat alone, Robinson says it hasn't been easy.
The girls were born at 31 weeks' gestation and, although Isla came home after another seven weeks, Lily stayed in a further six beyond that - and had a feeding tube for a year.
Robinson says, having never had children, that she was blissfully ignorant how difficult life with two would be.
"I worried, but only as much as I would have anyway. It would be worse if you'd had one before."
Her mother had to help virtually full-time at first. "At the beginning it's really difficult. A newborn is a lot of work anyway. With two, you just have to juggle everything."
Now the girls are getting to the age where having two is a lot of fun. But she says for other parents who find they are expecting twins, even Oscar-winning actresses, she can offer only one piece of advice.
"Hire a cleaner. You'll need one."
