One Vatican, two popes: Benedict's back.

A new page was written in Vatican history Thursday, when Pope Emeritus Benedict XVI returned to take up permanent residency in the city-state.

The unprecedented co-location of a reigning pope less than a 10-minute walk from a retired pope occurred as Benedict arrived at the newly renovated Mater Ecclesiae monastery.

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The pope emeritus made the short helicopter flight from the papal summer residence of Castel Gandolfo, where he had been staying since Feb. 28.

According to a statement from the Vatican, he was greeted by Pope Francis at the entrance to the monastery and the two men shared a short time of prayer in the building’s chapel.

In another unique arrangement, the Vatican has confirmed that Benedict will reside with his personal secretary, Monsignor Georg Gaenswein, who accompanied Benedict on his return to the Vatican. Gaenswein will also continue in his day job as head of the papal household, with responsibility for Pope Francis’s schedule.

As the Vatican enters uncharted territory, experts predict immediate attention will be focused on any differences that arise – or may even seem to arise – between the two men.

“If Benedict fulfills his pledge to live out the rest of his days ‘hidden from the world’, it will reduce the chance of misunderstandings, or even of drawing contrasts,” Vatican journalist Alessandro Speciale told NBC News.

The two pontiffs had met at Castel Gandolfo on March 23 for an informal and largely private face-to-face discussion, which a Vatican spokesman described as “a moment of profound and elevated communion.”

In April, Pope Francis, who had already earned the reputation of a potential reformer,  launched an eight-member review committee to consider the way the Vatican is run.  According to the Vatican’s deputy Secretary of State, Angelo Becciu, advice on how he should proceed is ‘already raining in’, but Francis has yet to meet with the committee members.

If tension develops tension between pro- and anti-reformists, Alessandro Speciale said it could prove awkward having a former pope in the mix. “What might happen - and would be very embarrassing for the Vatican and a huge problem for the church - is that Benedict might become a rallying point for those who oppose the reforms and simpler style advocated by Francis.  The  traditional trappings and pomp of the papacy was something that Ratzinger was bringing back into favor.”

Benedict described himself as “a simple pilgrim” as he stood down from the papacy, but he will continue to wear white robes.

His new accommodation, located inside the Vatican gardens, is a comparatively modern building, having been completed in 1994.

It was founded under Pope John Paul II as accommodation for a monastic group of nuns and a place of prayer. Refurbishment work on the building started in November; one former resident nun said the basement suffered from humidity

Related:

• Popes meet for historic lunch
• Benedict leaves Vatican for final time as pope
• Full Vatican coverage from NBC News