Two landscapes by Peter Paul Rubens intended as companion pieces are to be reunited for public display for the first time in more than 200 years thanks to a decision most thought improbable.

The National Gallery is loaning A View of Het Steen in the Early Morning to the Wallace Collection for reunification with Rainbow Landscape. Going the other way will be a Titian, allowing for the first time in more than four centuries the complete display of six paintings known collectively as the “poesie”.

The reciprocal agreements were announced on Monday and follow the recent announcement by the Wallace Collection that it would begin loaning works for the first time.

Gabriele Finaldi, the director of the National Gallery, praised the Wallace for taking the “landmark … momentous, I would say” decision to begin lending.

“We are hugely supportive of this because the general public is the beneficiary,” he said. “Things can happen now that couldn’t happen before that will be extraordinarily interesting and exciting for the public and scholars.”

A View of Het Steen in the Early Morning by Peter Paul Rubens. Photograph: National Gallery, London

The Rubens landscapes were painted while the Flemish artist was living in retirement in a beautiful property between Antwerp and Brussels. They were painted for his own enjoyment and remained with him until his death.

The two works were last together in public at a British Institution exhibition in 1815; and privately together for study purposes at the National Gallery’s conservation studios in the mid-1990s. “It was so secret, I was a curator at the time, that even I didn’t know,” said Finaldi of the conservation study.

“By bringing them together we will be able to see what Rubens intended. We’ll be able to see how the two pictures work together.”

The Wallace’s Titian, Perseus and Andromeda, will travel across London to the National Gallery for the latter’s big spring show Titian: Love, Desire, Death.

Other Titian loans from the Prado in Madrid, the Gardner Museum in Boston and the Duke of Wellington means the six mythological paintings he was commissioned to paint by Philip II of Spain will appear together.

Perseus and Andromeda is heading to the National Gallery for a major Titian show. Photograph: The Wallace Collection/PA

Bray said the Titian and Rubens events would be an “unprecedented moment in art history, made possible by the Wallace Collection’s decision to works for the first time”.

Recognised as one of the world’s finest collections of art, armoury, furniture and porcelain, the Wallace includes superstar paintings such as Frans Hals’ The Laughing Cavalier, Diego Velázquez’s The Lady with a Fan and Nicolas Poussin’s A Dance to the Music of Time.

It was always considered a closed collection, because of the terms of a will left by Lady Wallace, but a re-examination by lawyers and art experts recently overturned that – a decision endorsed by the government and the Charities Commission.

The Wallace said future loans and collaborations would happen on an “exceptional basis”. One of the next projects, a result of the loan change, could be an in-depth study of the French painter Jean-Honoré Fragonard.

Bray was asked whether the changes could open the door to restitution claims – for example, the collection’s magnificent Asante head, a gold trophy object made in what is present-day Ghana.

He said there had been no requests from Ghana but the collection would consider loaning it under the terms of the bequest to the nation – that items could not be deaccessioned (permanently removed from the collection) or sold.

• Titian: Love, Desire, Death is at the National Gallery in London from 16 March to 14 June. The Rubens will hang together at the Wallace Collection between next May and January 2021.