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8 October 2014 Last updated at 08:47 BST
Their parents Lucy and Andy Keher, of Blundellsands in Merseyside, heard giggling after bedtime and filmed them when they found the pair playing the game in their cots.
Mr Keher posted the video on Facebook and it soon went viral, with fans sharing it all over the world.

Main, 23, scored once in 14 appearances for Rovers this season.
He started his career at Darlington before moving to Middlesbrough in June 2011 and joining Doncaster in July 2014.
Main could make his debut for the Latics, who are five points adrift of safety in League One, in Tuesday's trip to Blackpool.
Find all the latest football transfers on our dedicated page.

Atherton, 28, became the first rider in history to complete a perfect downhill World Cup season and then won a fourth World Championship title a week later.
However, the downhill discipline is not an Olympic sport.
Atherton said: "I'm proud and content with what I have achieved. I don't need the validation of an Olympic medal."
Other British riders have changed disciplines in the pursuit of winning Olympic medals.
Mark Cavendish, the only member of the 2008 Beijing Olympic track team to miss out on a medal, returned to the velodrome for the 2016 Rio Games and was rewarded with silver in the omnium.
He quit this year's Tour de France early, after winning his 30th career stage in the three-week road race, to focus his attentions on Rio.
And four-time BMX world champion Shanaze Reade is also heading back to the more controlled environment of track cycling for the 2020 Tokyo Olympics.
Reade, who won world team sprint titles in 2007 and 2008 with Victoria Pendleton, missed out on Olympic BMX medals in 2008 and 2012 despite starting as favourite because of the "lottery" nature of the sport.
Atherton said: "I definitely used to want to switch to track at some point but in mountain biking you spend a lot of time outside in the mountains - I think I would miss that lifestyle.
"Mountain biking has a huge following and a life on its own without [downhill being included in] the Olympics."
The Trek factory rider has won 13 downhill World Cup races in a row, including all seven this year to win her fifth title.
Atherton could now target the two-day enduro races, which include both uphill and downhill sections.
"That would be a huge challenge, having the fitness to do that, but it is something that I would be pretty excited to give a go," she said.
And, after a year "beyond anything" she imagined, Atherton has returned home to a small village north of Aberystwyth for the off-season.
"It is surreal getting back into home life. It is pretty cool to bring the trophy home to this tiny village in the middle of Wales," she said.
"When you are at the top of your sport it is a stressful thing, it is hard to enjoy because you are so focused.
"I'm actually looking forward to a year enjoying racing a bit more and not having to prove myself is going to be nice."
Twins Molly and Megan Keher have become internet stars after nearly two million people watched a video of them secretly playing peekaboo.
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Oldham Athletic have signed Doncaster Rovers striker Curtis Main on loan until the end of the season.
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British world champion Rachel Atherton does not feel the need to go chasing Olympic medals despite dominating downhill mountain biking.