The event marked the first time the duchesses have been pictured together since Christmas Day.
However, while the palaces gave no explanation as to why they had issued the guidance now, there has been rising abuse of Kate, the Duchess of Cambridge, and Meghan, the Duchess of Sussex, since the former USA actress married Harry last May.
"Much of social media has become a sewer", she said on Twitter last month.
Dragon capsule heads for space station
They rushed there from Florida after watching the Dragon rocket into orbit early on Saturday from Nasa's Kennedy Space Centre. Ripley was joined in the ship's maiden flight by Little Earth - a plush toy which looks like the Earth with arms and legs.
Park wins 6th LPGA title in Singapore with final-round 64
Park had edged in front and a sealed the victory in style with a nerveless 25-foot putt on the 16th for a birdie four. Tied for third on 11-under were South Korea's Ko Jin Young and Spain's Azahara Munoz, who both had final rounds of 69.
Is the 'Cheesed Challenge' Bad for Your Kids?
The trend started to take off a week ago, but it is unclear who posted the first video which inspired the challenge. The peculiar prank had already taken on a life of its own, before the original poster chose to remove the video.
Rumours of a royal rift between the pair began swirling after news emerged that Meghan and Harry will move away from Kensington Palace to make their family home 25 miles away in Windsor.
"Prince Charles was created The Prince of Wales aged 9 on July 26th 1958", Clarence House tweeted.
The Duke and Duchess of Sussex, the Duke and Duchess of Cambridge and the Duchess of Cornwall, along with Prime Minister Theresa May, were among the guests at Buckingham Palace. On the other hand, Prince Harry and Markle were positioned at the back.
Britain's Catherine, Duchess of Cambridge is seen at a reception to mark the fiftieth anniversary of the investiture of the Prince of Wales at Buckingham Palace in London, Britain March 5, 2019.
Meghan cut a graceful figure in a recycled £895 Amanda Wakeley coat teamed with a floral embellished dress as she stood alongside Harry during the celebrations on Tuesday, joined by Kate and Prince William, 36.